It depends on what turbo your using Ive blown standard boxes on T34's with big boost spikes , but also have used a standard box on a UT T38 as it doesnt make the same torque and power comes in smoother.

MK - The gears kit does look like a decent upgrade for a road car that does not want the noise of a straight cut kit, but it depends if it has been properly tested in a big powered car.
Mark get them hit up for a discount your car would be ideal as its a high profile build,going to be good power and will get used hard. Im sure they did Imsy a good deal on his straight cut kit for his fiesta as he used it on track and tested the kit.