how much horsepower can the 904 take?

Transmission Durability and Longevity are both the results of keeping the fluid below 200* through whatever abuse it is put through.....Fluid reaches 230-250-280 and the transmissions life will be measured in minutes/seconds.

A more robust cooler than just the Engine Coolant Radiator is needed when power levels exceed 400 crank horsepower.

A stock 904 could handle a MP Crate 360 no problem.