Well, there are three ways to make your car faster.
1) Less weight
2) More power or
3) Handle/stop better
It all depends on how 'modification friendly' your insurance company is.
The only one that will have no impact is number 1 - there is nothing stopping you stripping out some sound proofing etc to lighten the car, and you won't need to tell your insurance company. The engine will have less to drag around, and hence it will be (probably almost imperceptably) quicker. But it will be nosier, which may add to the sensation of speed!! The further you go, the quicker it will get.
A replacement filter and exhaust is the common route to get half a dozen or so brake horse power (don't go expecting 5 seconds off your 0-60 time!) and most insurance companies won't pull your pants down too far for those mods (I think they realise its a pretty standard mod that most people wanna do, and doesn't increase the risk by too much).
The one that will probably cause the biggest increase in your premium (but will probably give the quickest real-world increase in average speed) is to improve the handling of the car and improve its ability to shed speed (ie brake harder, later).
Being able to carry 5-10 mph more through a bend, and braking 2 seconds later will make your car much quicker and much more fun.
Check with your insurance company to see what they will allow you to do!
