Definitely liking the Spec R geardiff so far. It builds up sooooo smooth. The fit and finish of the parts is good. I did notice a small amount of oil leaking show up on the pulley teeth. I am running the rear belt pretty tight to keep it from skipping. Ran about 8k oil (mostly 10k with some 5k mixed in) and it did not change the cornering characteristics much, but it gave the car a lot of snap off the line and out of tight corners.
The best thing is how smooth it is, and it STAYS smooth.
From what I have seen the leakage is from the center paper seal. I didn't notice any leakage from the outdrive. I have a second diff and I am considering putting some "gasket maker" from the auto parts store on the paper seal before putting the two diff halves together. Definitely buy a set of extra gears and a set of extra seals when you order the diff.