I agree that they would not need to test an engine they wouldn't use so I guess my thought is that they are more testing chassis and suspension dynamics. If that is the case I would think that any engine with similar weight would work. I do agree however that if the engine they are planning on is ready for primetime they will be testing with it. I don't see why however they wouldn't use a different engine for tests if the actual one is still having some tweeks. Remember we are still a year out and they mule were probably built 6-12 months ago so almost 2 years out. I think newer test cars could have a differnent engine that the "de-camoed" older mules.
Anyway just my thought I could be completly off base, I just don't want to admit that torque below 400 is possible for the V8 so that may be coloring my thought.