Is a compression test performed on an engine stand valid?
I am thinking of using an impact wrench to get it spinning.
Rich

Not really, the engine should be warm and I don't think the impact will spin it fast enough. A leak down test would give better results with the engine out of the car.

?????Spin your engine with an impact?????
Engines are tough but that would not be a good idea. The pounding can only hurt.
Find a good heavy duty 3/4" drill. Make an adapter for it and turn the engine over with that. You can still take compression readings with a cold engine but they will all be about 10-20% lower than a warm engine. What it will tell you is if you have different readings on different cylinders. A bad one will stand out and you know you will have to work on it.