Because for only 300bhp a T34 would be a lag monster, plain and simple.

A good 300bhp turbo could have full boost around 2-2500rpm. Will a T34 do that ?

The OP would need to be very very specific about his goals, and his driving style and use for the car.

I think what NMS are saying is that 300bhp is gutless, and I'd pretty much agree. Compared to more power, it is gutless lol But does the OP want fast spooling low rpm torque ? A nice relaxed spread of power ? Is it a light or heavy vehicle ? What sort of traction ?
Closest comparison I can make is driving a friends Rousesport with a small turbo. And having the small turbo actually made it very nice for normal driving.. Was it quick ? No, but it was nice to drive. Far better compared to a standard car with the ancient old T3 yet overall they had the same power ( Rouse might have been slightly more actually )

So what does the driver of the car actually want ?