Nicholas please do not take this the wrong way but....... What are you doing racing guys on the open road. Even with the motor only pulling 4350 rpm's your still doing 85 MPH. The standard Dastun brakes are pretty weak so look out if some idiot pulls out in front of you while you're going 85.
I don't know much about motorsport in your country but you definitly need to go to some sort of track day. Here in the USA we have great acces to track days,Racing,Rallycross and Autocross so street racing is kinda pointless. If you have access to these entry level events get to one right away , your in need of a fix.
As far as your motor problem , this is what a stock 1200 will pull regardless of wieght in fact it also sounds like your modded 1200 is even less aerodynamic than the stocker. It is possible the car is running to rich this will cause the motor to flatten out. I run a modified GX motor in my 1200 race car and at one of the fastest tracks in the USA it only pulled 6500 RPM's with a 4:11 final drive.
Tom