I think Inokinetic has some soft ones. I ran these for a while and they were softer than the yellow powerflex: https://www.elise-shop.com/toyota-engine-engine-mount-inserts-111r-exige-s2-elise-sc-elise-r-211-p-415083.htmlBumping this.
My intercooler duct cracked only one month after removing the rear poly inserts. Now I want to put them back in... but is there a softer unit than the Innovative? I remember hearing some have used just a half side of the insert, but that sounds like it would damage the stock rubber.
When I had my MR2 Turbo, Toyota had installed a LARGE rubber ballast weight on the subframe near the rear motor mount. That really helped with vibrations.