You can do all the engine mods you want and it won't change your top speed.
It will make it quicker (as is acceleration) but it won't make it faster.
Changing your gearing will do that. Drop one or two teeth on the rear sprocket.

 

I agree with Red, the quickest and cheapest way is to regear for more top speed running, if you are running at redline to try to get more speed, you will bounce off the rev limiter, especially with a modded motor, Honda puts a very low rev limit on the 300/400ex to keep the motor safe, any time you open up the air box or exhaust, the motor revs freer, and you hit the limit faster. A +1000 rpm CDI will definately help, or you could just regear for the rpm band you want. and..........

PLEASE SHUT OFF THE CAPS LOCK..........

 

you actually hit the rev limiter in 5th? how many miles does that take? the rev box will do nothing.