930 limited slip questions

This is for the 930 owners out there, or other Porsche people who know about limited slips. I've posted these questions to the Turbo Board, but haven't gotten any responses.

1. I own a 78 Euro 930. I have heard from some sources that these had limited slips on them, without exception. If mine has one, it's pretty worn. On a hoist, if I move one wheel, the other one doesn't move at all. If it moved in the same direction, I could be pretty confident there was a slip in the diff, right? And if it moved in the opposite direction, I could be pretty sure I had an open diff, right? Or do I have those two backwards? But if it (the other wheel)doesn't move at all, could that mean a worn diff that just needs a rebuild?

2. If I don't have a diff, should I get one? The engine's out now, so now is the time. My engine is not stock--it makes over 600hp and over 550 ft lbs at the crank at 1.1 bar. The car has been in Arizona at Powerhaus and Motorsport Design, and those guys tell me that when they drove the car before the engine came out, they didn't notice wheel spin at .8 bar. But I live in Seattle, where at least some wet driving is inevitable, and I don't want to be limited to .8 bar necessarily, anyway. I plan on a 80/20 street/track split up here.

3. If I should get a diff, and I don't already have one, what kind--Quaife or GT on the one hand, or factory clutch type, on the other? I am not, obviously, a "factory uber alles" type, so I have no prejudice one way or another from that angle. My research thus far indicates that a Quaife-type might be smoother in operation for a mostly street car. The two types seem to be roughly the same cost.

Any advice would be appreciated.

If you have 600HP you must either have huge tires out back or posi or both if you're going to get any traction at all. Surely you know if both tires go up in smoke when you stand on it. If your tires are huge and won't spin then you have no worries - posi is not a good thing in the rain.