Just replaced my broken std diff with a Q2 in my GT JTD which is chipped and putting out around 185 bhp with a big torque increase and thought I'd share my thoughts on it.
Traction is greatly improved, especially in the wet, the car tracks straight even under hard acceleration in slippery conditions, albeit with both wheels spinning instead of just the one, pulling out of junctions in the wet is much more low key especially on a hill, it's no longer difficult to avoid looking like a boy racer with the accompanying loss of traction.
I can now happily floor the loud pedal exiting a bend and the car just holds it's line and doesn't spin up and head for the scenery. The steering is altogether easier, white lines, surface irregularities and enthusiastic acceleration no longer see me making large adjustments to the wheel constantly.
In the dry the car feels pretty similar to standard but a little more settled and planted when driven hard.
On the whole a worthwhile improvement even if you don't press on very often. The diff comes with the peace of mind that you won't suffer the same fate as I did and have major surgery and a big bill on your hands when your std diff expires. It's also a relative bargain and as well as an upgrade should be seen as a preventative measure. The cars should've come with them as standard when fitted with the more powerful, high torque engines at least.
All in all, highly recommended