My understanding is that the Mercury Playback Engine helps with rendering effects and layering while editing. It does not help with the render process while exporting the finished sequence.
We had some previous generation HP xw workstations that were awesome back in the day. We put new Nvidia cards in them. Although it helped while editing, MPE was of no help in rendering (or multicam editing when using XDCAM). The final render seems to be CPU bound.
Mercury PLAYBACK Engine is a misnomer. It probably should have been called Mercury Effects Engine.