there are not really huge gains to be have with this bike, but you can get a few ponies more without going totally crazy with money, good mod candidates for that are:
aftermarket exhaust
air filter (usually people go for KN)
power commander V (pc-v)
proper dyno tuning
that would cost you something below 1000$ depending on the components you chose and could bring you to about 50 HP
depending on the legislation in your country, you can go with catalytic converter delete, i'm aware of this one
IXIL DECAT RACE PIPE HONDA CBR 500 R 2013-2016 - IXIL USA so that's another 170$
few people went with aftermarket cam shafts, didn't see any results from that yet
another good way to improve performance is what was already partially mentioned ... new tires (when the time is due), shocks, forks, and making the bike lighter by removing unnecessary stuff (passenger pegs and so on)
this won't increase the power, but will increase the power to weight ratio and change weight distribution, which in result will cause better bike handling (for example when i changed the 4200g heavy stock exhaust for the 2091g carbon fiber sc-project, i felt the difference, the bike was falling to a lean faster)
so basically ... it's up to you how much money you want to spend and what is your line when you say "okay, that's enough"