my .02 save your money. the HP youll gain is very little in respects to the ammount of money youll spend. also exhaust/intake upgrades often require rejetting of the carbs, as well as a reduced resale value. my advice to you is learn to master your ex, sell/trade it off for a 600 later.
ill be trading mine off in 2-3 years for something faster, but its far better to learn manuvering (not just normal driving) on a smaller/more forgiving bike than one with double the HP (like a zzr or 636) and a twitchier setup.
that being said, if you dont plan to upgrade and are happy with the ex and just lookin for a lil more pep and flash, go for it. from what iv read tho, sprockets....not exhaust are the quickest/cheapest way to change the performance of your bike.