John,
I would think that you would want to stay away from a locked rear end in a road course car. Unless you are running relatively tight turns and short track. Even then, Don't know that there will be any gains. Kind of a go kart approach if you know what I mean. Would cause a lot of wear on tires.
I am planning to run SCCA auto-cross. I'm thinking, due to the tight turns some of the courses use, that a locked rear may help in some situations.
I have a friend that ran road courses down south. If you have specific questions, I could ask his oppinion. He's military, and working on his schooling right now. His time is fairly limited; but I would pose your questions to him and pass along his answers.
There are some others on this site, that are also running venues other than ovals, that may be able to give you better answers. Post up your Track, set up, and any other info you can supply. PM me if you are not comfortable with that. I'll try to get you in the ball park.
Steve