It should first of all be noted that MS is one of the only companies with enough balls not to try to get 100/100 on acid 3 (many of the "requirements" are not part of any official and final specification). I see MS as being the *least* likely to cheat on pointless benchmarks.
Second, adding a return value *does* in fact make a huge difference in run time. There is a very large difference in the way the stack functions, and many optimizations can be made when there is no return value in a function. There are of course some other requirements for this to be possible, but I'm going to go ahead and assume that the JS engine is just being smart about running pointless code. This is not cheating. Perhaps they were encouraged to make this optimization in order to do better on the test, but it could still very well be a valid optimization.