the bushings that are rumored to be changed are the front control arm and rear twist beam, not the motor mount. As for the RMM, the modification does make shifting much smoother and reduces the wheel hop, the stock motor mount isn't nearly as bad as everyone says. I drove a prototype 2014 ST for 20,000 miles and almost all of those miles were very hard. The car was pushed at the track, autocrossed until the tires gave up, driven off road, and generally hooned harder than I would ever do to my own car. Never once did the car give any problems. I doubt the claims that the engine hits the firewall. There is a sound that isn't pleseant when shifting very hard, but I think it is more to do with wheel hop. There is a good deal of clearance between the motor and firewall and the bushing movement couldn't possible be more than .25-.38". The bushing core material can only compress so far and that isn't enough to cause an impact between the engine and the chassis. I did install the Cobb RMM on my personal ST because I like the way it improves shifting, reduces wheel hop, and reduces the fore-aft motion of the engine under shifting and load (which is why it helps shifting).

Cobb Fiesta ST Rear Motor Mount by
Bryan Redeker BRGT350, on Flickr
You can see here that the bushing material in the stock RMM can't compress very far before it locks out. The Cobb RMM has much less movement due to the bushing material.