I just got a DG Bullet slip-on Friday, and man, did that wake it up. Re-jetted the carb and put on the pipe, and it is a whole new machine... not a bad price either.

 

OK so you probably want a pipe that mainly increase top end power. And you said you just want a slip on for now. So here's my suggestion:
Find an article that has done a shootout, and hopefully has dyno charts. Look for the ones with the best top end.
and
look for a slip on that you can add the header to later. You can not add the header on a lot of them. With the HMF, you just gotta cut off the little reducer. The headers mostly add top end, so this will probably be pretty important to you.

 

get a ron woods for draggin the yfz, awsome pipe.