I just figured I'd post up my personal experience here, to inform the rest of you with what I've ran into. On the stock exhaust, with the stock tuning...I was able to do some fairly substantial modifications to the airbox without any problems. It seemed to help the upper rpms out quite a bit.
However, since I've gone to the new exhaust setup and had the carburetors retuned...honestly, the bike runs MUCH worse with any airbox mods done. I had one hole per side of the airbox dremeled out...large ovals, about one inch by two inches, and it ran terrible---flat spots everywhere. I predicted this...so then I covered up the right side completely. Got better, but still flat spots. Then I covered up the left side half-way. Better, but still had a flat spot in the mid range. So finally, I covered up the left side hole completely. This left me with a COMPLETELY STOCK airbox, minus the trimming I did to the pair of airhorns. And guess what? Flat spots disappeared, and the bike picked up power EVERYWHERE in the powerband.
So the moral of this story is, I guess....airbox mods are ok for stock. But if you start getting mods into your EX500....be careful what you do to it, it may just run worse.
Currently, I'm running the stock airbox, with a K&N drop in, and trimmed down airhorns. And the airbox is now covered in duct tape, to mask the holes.
