Chimp,
Any time you open up air flow (in or out) you generally need to add fuel, especially if you don't want it to sound like a popcorn popper. So yes, a rejet is necessary. The difference is dramatic though. After I did my pipes the first time I didn't rejet. The performance gain was minimal and even with the AI removed it popped a lot on decel. This spring I decided to do the airbox and rejet at the same time. My first attempt was just to pull the snorkel and drop in the UNI. It made a very noticable difference. I probably would have been happy, but it still popped a little and I could tell the bike still wanted more air. I modded the airbox and *holy crap* did it make a difference. It was able to dial out the popping much more easily and the power increase rocked.
Doing mods like intake, exhaust, and a rejet job aren't really hard on an engine, so there isn't much of a compromise (except for the tendency to ride harder). Rejetting takes about an hour or two the first time you do it. It's really not nearly as hard or scary as one may think, especially if you follow Dinqua's
directions on his website. It was my first time inside of a carb and I was a little intimidated at first. If you try in and run into trouble, I could certainly stop by and help. I've rejetted a few times and even drilled holes in my slides, so I'm not intimidated anymore.
