Installed a few things today to tighten up the transmission and rear diff:
Perrin Brass Shifter Bushing - PSP-INR-016 (also requires 2015 WRX transmission washer, Subaru P/N 031208006)
Whiteline Positive Shift Kit - KDT901
Whiteline Rear Diff Inserts -
KDT903 KDT927
STI Group N Transmission Mount (5mt) - D1010FE020
Perrin Short Shifter Adapter - PSP-INR-201
Shifting has definitely been improved. The short shift adapter definitely isn't for the faint hearted, it shortens the shifts quite a bit (30%ish) and the gate feeling is very notchy (think 'snick-snick' into gears).
The STI transmission mount and whiteline positive shift kit make shifting much more engaging, you can feel exactly what the car is doing now instead of having to guess. I noticed a slight increase in NVH but nothing more than I was expecting. A little bit of extra vibration in the cabin while sitting at stop lights, and 1st & 2nd gear whine like a straight cut box now. I like the whine but some people may not.
The rear diff inserts have noticeably improved the low speed driving, the car hardly jerks at all in carparks etc, and when accelerating the hesitation is all but gone. A note with the diff insert kit is that the inserts for the top of the diff cradle fit, however the MY10 GT's already have solid bushes in the front rear diff cradle so the other set of inserts included in the kit weren't needed. I gave mine to the member who did the install with me as they will fit his GenIV.
While the car was up on the hoist, we also changed my 22mm rear swaybar to it's stiffest setting. All in all I'm quite happy with the combination of parts and now I'm looking forward to getting the rear crossmember inserts in to tighten up the rear end!
Pics to come, it's late and I can't be bothered resizing them for imgur...
Edit: Changed KDT903 to KDT927 (this is the proper part number, turns out I bought the wrong kit, that's why one set of bushes didn't fit.)