faster gpz 500s

unless you have a big bucket of money not a massive amount.

Full exhaust system,
Free breathing air filter,
A bit of dyno time and a jetting kit
anything further to the engine will cost quite a bit

Get your suspension fully serviced and possibly upgraded to work specifically with your riding style and weight, that will make a massive difference, far more than adding 5bhp with the zorst/filter

Upgrade your brakes and service the callipers, braided lines and decent pads: if you can brake later you can come off the throttle later

play with sprockets, adding teeth to the rear or removing them from the front will give you more acceleration and if the bike currently doesn’t rev to the red line in top will actually give you more top speed.
remove teeth from the rear or adding to the front (if you can currently hit the redline in top) will give you more top speed but will mean you will lose acceleration.

Generally 1 tooth change on the front is roughly the same as 2.5 change on the rear, front sprockets are cheaper too.

Easiest way to go faster is to lose weight, either go on a diet yourself if you could lose a few pounds or strip bits off the bike that are not needed/don’t aid aerodynamics. Lighter fairings are an option as well, the rear subframe will weigh a tonne but is also structural and modifying it will potentially remove your ability to carry pillion passengers as it wont be able to support the weight.
Track fairings are lighter than the OE ones, you also will then be able to remove your lights saving more weight but losing your ability to ride at dusk/night time.