More Performance

If your looking for better performance from your ZZR there's not really much you can do without spending large wads of cash. This is due to the fact that most 250's are quite well refined to begin with. Just remember stock 250's are always worth more than any mod you can add to it.

Exhaust Systems

Changing the exhaust system on the ZZR250 will only change the sound (so I've heard). No pun intended. I've read of one rider who changed to a 2into1 manifold into a single can. apparently it sounds like a 600 and can gain a little at the top end once tuned to the new pipe setup. if tuned correctly you should not loose out in the bottom end. How much did it cost? I don't know. is it worth it? well, if you have the money to spend on it, and you don't mind loosing out when selling it... sure why not. Just remember if you change to a 2into1 pipes and can you'll need to re-jet.

Air-Intake

You can get some K&N air filters that suit (Dual Flange Oval Universal Filter) and it will give you some top end however this will come at the cost of mid-range power. In addition you might want to add a 2into1 exhaust, re-jet and tune. Or buy a 600!

Turbos Vs Superchargers Vs NOS

If you want to spend the money and don't care about resale value, try these.

I've calculated you can increase the Kilowatts from 27.9 to around 57 by running 5 pound boost. The question now is, how to deliver the extra HP? 

The cheapest and most efficient way, I think,  would be a supercharger running a constant 5 pound boost. The extra power is always there if you need it and you don't loose so much midrange as you would running a turbo. Remember the 250 is a small capacity engine so even though you'll get a nice HP boost when the turbo kicks in, you will loose lots of power when the turbo is not active.

The final and most expensive method is NOS. I'm not going to go on about NOS because your better off getting a 600 in all three cases anyway, unless you want the only supercharged 250 out there! please understand that adding such performance parts to your 250 will run it into the ground and I haven't mentioned spending additional money on appropriate cam chain, pistons, con-rods, exhaust manifold, re-jet etc.