Just installed my gear diffs on my TRF501. The diffs are manufactured by Pumilio Design. I have not yet driven them, but they feel very smooth, hopefully they will perform too
On the down-side, at least on my car (TRF501) the front upper deck needed some cutting for the diff to fit, but at least on my opinion, it is not that big of an issue.
Pic.
The price is 120 eur for one diff, which equals to slightly more than a new ball diff made with ceramic bearings and balls.