Here is a video of my bike's throttle response.  I can snap it open pretty fast and there is instant response.  That's with a #8 slide and an "EGK" needle.  A #7 slide should be better.

The video also shows how the bike tolerates a chopped throttle.  Note that towards the end of the video, I repeatedly accelerate and chop the throttle.  No backfires, no kabooms.  You shouldn't have to hold the throttle open or set the idle extra high in order to keep the thing from backfiring.  I set my idle at about 1300 rpm, and when I decelerate, I simply chop the throttle completely.

As Dave mentioned, you can see that the fuel air mixture goes grossly lean when I decelerate, but there's no backfire.  I don't know exactly why these bikes backfire, but my experience has been that the slide needle plays a bigger role than the pilot jet or the idle mixture screw.  Again, I can't explain why, it's simply been my experience that when I get the slide needle squared away the backfire is gone.  That has been my experience with the PWKs, the VM, and the stock carb.  

https://www.youtube.com/watch?v=1SEvGojazP8

I forgot to mention, the DGK needle is a leaner needle than the DDJ.  I must clarify.  The DGK is leaner from 1/8 to 1/4 throttle, but richer from 1/4 to 3/4 throttle.  DBM 8/1/23   It's a knockoff so no way for you to know if the as-delivered needle is in fact a DDJ.  The DGK has a slightly larger basic diameter (2.695 vs 2.685), and the taper starts farther down the needle (39.95 vs 36.80).  Both needles have the same taper (1 degree 45 seconds minutes).  This link shows how the needle designations work.  It tells you what each letter in the designation stands for (angle, L1, basic diameter).

https://jetsrus.com/a_jets_by_carburetor_type/needle_keihin_N427-48.html

I believe you mentioned that you have the needle clip in the top groove.  That should be pretty lean.  I would be careful with that.

My comment on the DDJ vs DGK was simply to let you know where the two needles fall on the lean/rich scale.  The link to the needle chart should be useful.  It's very important to know where a particular component falls on the lean/rich scale so you can arrive at an informed assessment of any particular change.  If you install a needle that you believe is richer, when in fact it is leaner, you will obviously arrive at an incorrect assessment of how the change worked and what to try next. That's why I previously warned you about cheap jet kits.  

It's hard to express in words how an engine is running, and you & I use different terminology to describe things.  At this point, I have no idea if you are rich or lean.  I simply want you to be aware of where a particular part falls on the lean/rich scale.

Not having solid confirmation regarding your main air bleed, your needle jet, and your slide cutaway makes any sort of assessment close to impossible.

What about the clamp on your manifold?  Did you install a good clamp or is the stock clamp on there?

I must apologize, I gave you some semi-bogus info.  I have been struggling with your problem, trying to understand what is going on with your bike.  As a result, I keep going through needle charts and my old notes.  Last night I realized I had given you some misleading info.

I told you the DGK was a leaner needle (leaner than the DDJ).  That is not exactly correct.

The DGK is leaner at 1/8 to 1/4 throttle, but it is richer at 1/4 to 3/4 throttle.  That's important since your problem occurs at the higher throttle openings.  I have to correct my post.

Your concern over being too lean is valid.  You can hurt it with a lean mixture.

One thing I noted on my bike was that the DGK needle had a tendency to control fuel delivery at wide open throttle.  Typically, you want the main jet to be the controlling orifice at WOT.  The needle and needle jet should be out of the picture by the time you get to WOT.  That's why I switched to the EGK needle.  The EGK has a steeper taper (2 degrees vs 1 degree 45 minutes).  The EGK is richer than the DGK from 1/4 to 3/4 throttle.  Note that the needle chart only shows the needle's effect from 1/8 to 3/4 throttle (1/8 to 1/4 horizontal top bar, 1/4 to 3/4 vertical side bar).  The needle should drop out of the picture past 3/4 throttle, and the main jet should be controlling the fuel delivery.  But you seem to be affecting your WOT mixture when you drop the needle position by installing the e-clip in the top groove.  That indicates that your needle is controlling WOT mixture.

Your baffle seems very restrictive.  You mentioned there are a number of these plates over approximately a ten or twelve inch section of the pipe.  The pipe ahead of the baffle has very little volume.  There's no place for the hot exhaust gas to go.  It just backs up in the pipe.

Mike, thanks for taking the time to look into my questions like that. I really appreciate it.

What about the DGK led to it metering fuel at WOT? That's something I'd like to avoid.

I'll just clarify the current situation.

The issue isn't present at WOT. Sorry if my wording made it appear this way. It is present at 3/4 (maybe slightly past) while at higher revs.

Currently im running the 140 main and 40 pilot. The bike is great to ride with either the DDJ or DGK needle. However, they both seem to be too rich at their final taper.

This presents itself as great performance at WOT (no backfire or issues at WOT at all) and all through the rev range until just before the main jet takes over from the needle jet. At this point (and to be clear, I can deal with this issue and have fun riding regardless) it starts to break up and sputter, but the sputter  stops if I increase or decrease the throttle.
So to get this problem to appear on needle clip position 2, I have to hunt for it. Position 3 it's present over a wider section of the throttle range, 4 more so etc, with position 1 having it not present at all but being too lean all round.

I messaged Sudco and got this response

"The first letter is the taper angle, which affects more 1/2 - 3/4 throttle.
The second letter is the L1 length which affects more 1/4 - 1/2 throttle, like clip position.
The last letter is the straight portion diameter before the taper starts, which affects more 1/8 - 1/4 throttle.
They overlap a little but that is where there primary affect.

If you want a leaner taper than EGK then that would be DGK. If you wanted a leaner taper than DDJ, then that would be CDJ but we don't import that one, so there would be CCJ or CEJ. CCJ would be a half clip richer than CDJ and CEJ would be 1.5 clips leaner than CDJ.

The PWK carburetors are designed for 2 strokes. 2 strokes require more fuel also require oil to go through the carburetor as well so the passages are larger than a 4 stroke carburetor. When people try to run the PWK on 4 strokes they have trouble getting them lean enough to function well in all circuits"

I just did a quick ride with the slip on tail pipe removed. Performance was slightly better over all and I couldn't find the 3/4 throttle stutter.
Maybe the stock header and crappy tail pipe are the issue. I might try removing some of the baffles