hey , i'm not trying to minimize the 500 or it's potential . a stock t100r was rated at 39 hp .
if you're used to riding your gixxer yea you might be disapointed .
first thing i'd do is make sure the bike is in top proper tune and that includes the current internals such as pistons/rings/bore and head valves/seats/guides ,then decide how much money you want to spend .
easy bolt-ons would be free-flow air filters and free flow exhaust .
after that open up your wallet .
big-bore kit . the only one i know of for the 500 will increase it from 490cc to 530 , not the 100cc increase of the 650/750 kits and cost @ $600 not including install which i believe includes machining the cases (that after the engines been disassembled) .
lighter valve gear , yup a few hp there too . kibblewhite makes a 7mm kit valves , guides retainers etc at @ $350 not inc machine work reassembly .
when you get into porting, cams , carb setup unless you're really experienced at that kind of tuning you're going paying someone else hours of time dialing it in . definately not cheap and the results might give you a great track bike , but one not so great on the street .
hey why would i minimize the greatest motorcycle ever made

?
sorry if my earlier replies gave that impression .
my advise about making sure the bike is in proper tune first still stands . timing ,carbs ,valves oh and if the bottom end has't been apart in a few years , the sludge trap before you do anything else .
cheers , Woody