Been riding a rickety stock '93 GS for a few months, finally popped in 40 pilots and 127.5 mains with a K&N in the airbox, no restrictor.  Also rigged up a carb syn tool w/ vinyl tubing, yard stick, and 2-cycle oil (as per suggestions on this site).  Haven't taken a long ride with the changes, but down the block and back made a world of difference!

Overall, everything feels more smooth - the shifts and the ride; and the powerband is much more even.

COSTS:
40 pilots (2) - $10.60
127.5 mains (2) - FREE (traded them clean for pair of un-used 140 mains that came in a FactoryPro kit) - ps. so angry i wasted money on that kits - basically $80 for adjustable needles...just shim the stock ones!!! grrrrrr)
K&N - $40
Sync-o-nator (??) - $4
TOTAL: $54.60
-your GS no longer feeling like a moped - PRICELE$$ 



PLEASE RESPOND:  I went with 127.5 mains b/c the shop was out of 125 mains and according to the jetting matrix, both work. (http://cgi.stanford.edu/~sanjayd/gs500/Upgrades/RejettingOld)

Would 125 mains be more productive in this setup???  Not sure if I feel like ripping everyting apart again to swap out 127.5 for 125's unless it will make a noticable difference.

Also, my other GS with K&N pod, V&H exhaust, 40 pilots, and 142.5 mains is giving me headaches - lots of random high-revving and backfires after warmed up - HELP!

http://www.motorcyclecruiser.com/tech/motorcycle_jet_kits_carburetion/

Check this link, it should help. You should be fine with the 127.5s. If its running right its fine.
As far as your other one, your main is lean, go 150. Have you raised the needles? Check that link out and go back to the bike, its easy to figure out, good luck!

K&N SU-5589 (in airbox - just like stock without restrictor and a pipe = 127.5.
You'd need 150's in that bike that got 142.5's. Pipe and pod = RU 2970 right - no airbox and clamp on K&N right. Yea 150.
Cool.
Srinath.

Yeah how about that - I figured 150's would be needed - but the stupid FactoryPro 3.0 kit (which cost $90, and it's made for use without airbox & high-flow aftermarket exhaust) only included a pair of clip-adjustable needles, a pair of 140 main, and a pair of 142.5 mains - how crap is that deal!  I wish I had known what I was doing before purchasing a "kit" with basically no useful parts - any idaes on a way to warn newcomers before its to late?

I'll update once I install 150 mains - thank you!

Move the clip on the needles also