You may want to get in contact with APR about this or even audi. The 4.2 is good for a lot more then 4whp fully built but it could be very hard to find anything for it at they are still using that motor in GT2-3 class cars. But fully built i think even if you keep it at 4.2 and dont up the displacement you could see more then 400. The R8 4.2 makes 430 from everything i can find on it. So that should be your answer right there.