Replace the engine, transmission and subframe bushing (at least the lower with a Powerflex hybrid) at the same time. If you don't, you're not solving any problems. Do not reinforce the rubber subframe bushings with metal inserts, this is a bad idea.
"The whole engine and gearbox (the drivetrain) is located in just three places. The two upper mounts – one at the end of the engine, another at the end of the gearbox – both of which support the entire weight of the engine/gearbox, plus a third Lower Subframe Mount underneath the engine to prevent too much fore-aft movement (often called the pendulum, dogbone or subframe mount).
The GTI uses very compliant rubber in these mounts to give a soft take-up of power and isolate the cabin from drivetrain noise. For a daily-driven stock car, these bushings do a great job of keeping cabin vibration to a minimum, and cushion the effect of bad gear changes or clumsy throttle applications from ‘normal’ driving. But when you ask more of these mounts, whether through higher power or harder driving, the movement in the mounts becomes very noticeable.
This gives a slightly loose feeling to the drivetrain when coming on and off-throttle, and an increased chance of wheel-hop under hard acceleration as the whole engine and ‘box assembly judders – sometimes violently, particularly with wheelspin. If your car produces high power, or has covered quite a few miles, or is driven hard, then the softness of the standard rubber mounts allow for more and more fore/aft movement as you come on and off the power."