Upgrade. They give a much better feel over stock. Still the best thing I've heard to increase handling is to adjust the camber both front and rear. For the cost of the sway bar kit and a Cusco front strut bar there is no reason not to upgrade. Thats all I have on my 300zx and it made a nice difference. Still I'll probably do the camber adjustments next.
I could be wrong on this as I know you are much more knowledgable about these Z's than I but I believe you are wrong. The LSD does act as an open diff during regular turns but when pushing through a turn at fast speeds where the tires would break lose on an open diff the LSD keeps traction and pulls you through. Honestly that was one of the first things I noticed when I got mine on my 280z. On turns that I would get the normal tire spin and back end wanting to come out now with the LSD I can not only take the turn faster but the back end just hugs the corner. It doesn't really help in long twisty roads but on sharp turns it does help. I guess it depends on what kind of curves you plan on taking.
clutch LSDs have what is called breakaway torque. on the stock differential it is some ungodly low number like 35-40 ft/lbs. When one of the wheels is appling more torque to the diff then the other, the power is driven to that wheel only, the other wheel "breaks" away and is allowed to rotate at a different speed, but resistance is given from the clutch packs. If this did not happen you would have a full locking diff and you would have chattering as the outer wheel was skipping along. sorry mister T.