Tire width, compound and weather conditions dictate possible grip... Driving style and low end torque, which we got, will dictate how easy you can break that grip. At least that's what I've learned and I'd be happy to hear a correction.
I got the Sport package on my Z so I got a Viscous LSD but it's an OEM one. I hear Quaife LSDs are good.
Since you're pretty close to me I'd recommend contacting IntecRacing for advice. I see a bunch of people from the Portland area coming up for tunes and such. Also I go there solely. Great people and they know what they're talking about. Especially considering they deal with slip a lot with the drift drivers they sponsor.