Motor mounts are in general meant to "temporarily" give to various stresses. Rubber's resilience (give and not break) makes it a good absorber of "slight" movements.
I am not sure what "spongy" means but if the mounts return to standard position, your PSS seal compression should too.
I would think "hardened or cracked" mounts would allow a compression shift without return.
Jim...
PS: I would guess a bit more seal flush flow in reverse, but I am sure PSS designed their bellows (seal compression spring) to accommodate both thrusts.