.
A proper encased, not just a poly insert, lower engine mount will do wonders in helping transfer power to the road. But for even balance it would be advisable to do the other two engine mounts as well. Every suspension related component will contribute to a good handling package, but it's a slippery slope!
Quaife, or other brand, diff is a major help to handling, in my opinion.
I've done most of the handling mods on the Mk5 GTI gradually over a period of time and can put my hand on my heart and say that it's a whole package which gets you to the sweet spot. My car now handles like a dream - It just needs a more skilled driver!