Wet street aside. I feel that an LSD is a requirement for any serious sports car. What's the point of building something and just spinning the inside tire in a turn. Adding an LSD to my cars has been (next to tires) the single biggest improvement in performance. The more twisty the course the more it helps. On the other hand, if you're just building a street car, don't bother.
Explain the fact that the Lotus Elise gets slower with the addition of a LSD then? That's a serious sports car and it does better without one, even the Lotus engineers agree.
And Ol'rowdy I have no idea what you are talking about in your anti open diff scenario above. Carguy makes excellent points about why an open diff is safer for the average driver. You might want to better explain that "situation" you describe where you loose forward mo and both tires break loose because of the open diff. I've never heard of or experienced anything like that. IMHO The more likely scenario with an LSD is that you are mid turn and momentarily break traction with the rear from a wet spot, gravel, poor throttle modulation or what not and BAM!- you're now sideways. With an open diff, only one tire breaks loose, usually the inside one (in a turn) which leaves the outside tire with no forces to deal with other than control of lateral roadholding. (which increases cornering stability) I've driven and abused vehicles of both types and I've never experienced what you describe. The open diff to me acts like a "governor" that doesn't let you put enough power down to step the rear out unless you are already at the limit of traction, in which case a LSD would've had you sideways long ago.
What I have found in a nutshell is that a LSD will cause a bit more understeer on track, (hurts on corner entry and helps on corner exit) will make it more likely for you to spin your car out and loose control when you don't want to, and will enable you to spin it out in a semi controlled manner and drift it when you do. Not to mention it helps on slick roads and what not, but who drives their 7s in these conditions anyway. (besides Jack and G
) An open diff really isn't a curse and a LSD isn't a necessity. My advice is build it with what you got and if you get adventurous later then go ahead and install an LSD.