I am actually looking into the same highflow option through GCG.
However the info that I received from GCG is:
If you are going stage 1 then you can bolt it straight on without changing anything else however this will not perform to its maximum efficiency. If you are going for stage 2 then you need to do fuel, fmic, computers, etc
The standard size are 320cc i think, anything bigger will be better. i.e. 550cc would do the job nicely.
You could also look at getting the injectors high flowed.
You will also need a new computer. I see you already have a aftermarket computer so you will need to retune it.
Hope this helps