If you trust Powerflex on the motor mounts, why don’t you trust Powerflex not to sell something useless on the subframe mounts? Powerflex makes subframe bushing inserts that are inserts just like these. Yes, the top and bottom are large and round, both to hold the vertical inserts and to fill additional bushing void and to give the bushing less room to move.
https://www.ecstuning.com/b-powerfle...yABEgI9O_D_BwE