This may not apply to most transmission mounts on other cars.
But this is what I found, By chance, I happen to weigh the stock rubber transmission mount against the Polyurethane repro mounts on the market by Imperial services and found that the stock mount weighed 1/2 pound more than the Poly mount. It was a pleasant surprise.

I've tried to call Imperial services with no success to see if they (since their mounts are made in house) would use my supplied shaped Titanium trans mount bracket (the one you see in the pictures) and install it into the poly mold to end with a super lightweight mount. Oh well, they don't return or answer their calls.