Glenn:
I've liked my Gurads Trans. Torque-Biasing Diff. ever since I installed it (and I put it in for the same reasons you're looking at). Better exit speeds, more predictable drift and over-steer "catch." The Quaiff's are said to be "Torsen" type torque-biasing diff.'s too, but I'm not sure about it. I went with the GT for cost reasons, and I'm happy with the cost and performance of their prduct.
You might contact Jae Lee at Mirage and discuss the heating effect of different differentials in different cars. I recall him talking about certain differentials causing heating issues. I know my GT TB diff. gets the gearbox pretty hot, so I run a synthetic Super Purple or Redline 75-90 wt. lube in the box. I was told if I were to exclusively track the car, to run a 85-110 vis. lube. So, I think there's a possibility of a heat issue. That's the only downside I know of with TB diff's.
I used to have a limited slip diff in my Jeep, that required changing the little clutches on either side of the sub-axles/ring-and-pinion inside the differential. I don't know if this is true of a "limited slip" diff. designed for a Porsche gearbox, but if it is, and the ring/pinion/diff is buried in the center of the transaxle, replacing those little discs could be costly. But, again, I don't know the mechanical workings of a P-car transaxle as well as my old Jeep trans and independent differentials.
Call Jae or John Rickard, or Steve G.. Bend their ear, and get the difinitive data from the pro's in the know.
And enjoy the improved handling. With TB diff's, it's great!
Hope this helps
Dan Chambers"It's
just a "well prepared" street car ... or a very, very well-mannered track car."
1983 SC #91 3.6L, "Black Pearl" Livery
1987 944 (gone but not forgotten)