Have you ever considered making actual solid mounts?
I have solid metal mounts on my Stealth and they really shored up the front end and helped wheel hop. There is a little more vibration, but it is almost unnoticeable after a day or so.
Not sure how applicable this would be on the W body platform though.
I just don't see solid mounts. Especially on a aluminum subframe. I think with a aluminum block the engine needs a little bit of play
06 GXP | 222/227 cam/cartuning turbo kit on 8psi/meth/e85 coilovers/ still on stock trans at 130k