The cts stage 2+ box is a good box i have one in my s2, am running 210/230 at under a bar of boost and its been great! all depends how you drive it launching won't do it any favours at all as with any gearbox. mine grips in 2nd after a short while lol then pulls hard through all the gears and it loves it.

If your going to be running 300bhp you really need to look into other mods such as the arb, adjustable tca's etc to really get the power down otherwise its useless mate.