I've tried about as many combinations as anyone, but don't consider myself an expert. I think KLR650 has those answers. But from experience I can tell you low end performance will not be as good and the engine low speed smoothness and drivablity will suffer with a straight pipe w/o a baffle. I have tried stock, a SuperTrapp with 4 discs, a straight Highway Hawk drag pipe, and ended up using a used RAASK drag pipe for a head pipe with one foot cut off and a HD FXD baffled glasspac for a muffler. Years ago I had a formula for torque improvement and longer was better for torque where short is better for high RPM HP. It has good sound and good performance. I runnning a stock aircleaner but the dealer has done some jetting for the previous owner so I'm not sure what jets are in the carb, I tried removing the white spacer and it was was too rich so it went back in. I do think the pilot jet yet may have been changed as low and mid range perfromance is great with the mxture screw less than 2 turns out. We just did the exhaust on my B-i-L's 878 with a direct rather than CV carb, and we changed from a baffle punched sportster muffler to a baffled muffler from the set with the bent extension. It looks great, sounds good and doesn't leak. Haven't had weather to test and fine tune it yet but at most I'd expect to maybe lower the needle one notch in the carb. It does have a cone type air filter on it.

if you open up the exhaust for more air flow, you MUST adjust the intake for more air flow, swiss cheese the air box cover (hidden behind the metal side cover), remove the air horn, k&n insert filter, round clamp-on air filter, seem to be the most popular mods. more air through the stock carb will not have enough fuel in the a/f mixture, so we increase the fuel by up sizing the jets and fuel metering devices inside. 152.5 or 155 main jet, 1/2 spacer on metering needle, removing the slug covering the idle mix screw, seem to be the most significant mods.
 i opted for the swiss cheese air box cover, stock air filter element, 155 main jet, 2/3 spacer, un covered idle mix screw, 17" harley sportster straight through muffler, i am pleased with the results so far, i intend to up size the pilot jet soon. there is some great tech info on carb flow found with this sites search. i hope that helps

Thanx KLX thats exactly the thread I was looking for. I really like the way my bike feels and runs now with just the idle screw adjustment and the stock exhuast with the hole drilled in. It's really quick to 70+ an tops out somewhere above my chicken level of 85, no bogging or anything but I really don't like the look and I hate that sound. Like a riding mower with no muffler! Just thought maybe with the straight pipe and the still to do white spacer mod I could eliminate the backfire when slowing down, clean up the look of the bike and add a little better sound at the same time. Plus I love the smooth line created by the pipe with no muff, same O.D. all the way to the end. Kinda like the pipes on the '05 M95 Boulevard. http://www.suzukicycles.com/Products/M95K5/Default.aspx If there are any muffs that will make mine look that smooth I'd rather go that route.