The 944 engine bay is probably not wide enough. I did see a 944 tub with a 911 motor. Anything is possible, but they may have had to tube frame the front end. 928 motors are too wide and also require a tube frame front end. Chevy V8's will fit with relative ease and given the cost, effort and performance gains probably the best way to go. Really what good is it to replacea 2.5L 4 with a 2.0L flat four? You can get 500 hp from the 2.5L 4 using a good turbo the same way you can from a subie motor.
If you want big power with little effort go with a chevy V8. If you want boosted power get a 944 turbo and play with that. One thing to consdier is that running 400+ hp will requrire alot of updates to the chassis to handle that power. You can get an easy 300 whp from a 944 Turbo with very limited chassis updates and if done right a very reliable powerplant. At 350 whp things begin to change and at 400 whp the stress you put on the chassis requires attention. The parts are there to upgrade, but not doing so ruins the balance and reliablity of the car.