52.5 w/holes, 1 3/4 turns, 150.

With stock exhaust, fine coming up from idle, response seems to flatten a bit at around 2800 to 3300 rpm, then goes nicely above 4000 rpm. Not fabulous, but I suspect the stock exhaust (soon to go away) isn't helping out.  

I am interpreting this as having a good idle mix, a very slightly lean pilot, and a good main.  Coming off idle I get the power and as the pilot takes more and more of the load, the mix leans just a little too much, then with more air the main starts to pull in and gives me a decent mix.

Is this correct thinking?  

If so, I may ask my dad if he's ever going to use his watchmaker's reamers.  They're very very precision.  I can squeek the pilot opening a tiny (and I mean tiny) bit wider with the reamer.  Think normal machinist's reamer on a very tiny and very long taper.  

I'm pretty sure this will work.  I see people looking for that 53.5 jet.  No trouble to make one.  We used to do this type of thing all the time in cars.  I am one of the few with a real SK carb wrench in my toolbox!  

Makes me wonder about altitude.  I used to have to stop and adjust my Fiat at 3000 ft or it would become unhappy.  

Just making sure my thoughts are right.  Maybe I could make 53.5 equivalent jets for folks.  All it takes is having the two adjacent sizes available, running a reamer into each, and then reaming smaller sizes to 1/2 way in between.  The jeweler's reamers cut very clean!

Sounds to me like you need to lose some of the white spacer on the needle.