I think it's like you said Five-O, the real benefit of the LSD will only become apparent when either ragging it round corners, or in very wet weather when negotiating a bend, when the inside wheel will want to spin.
The XDS system is controlled by the ABS braking system and effectively brakes the spinning wheel in an attempt to get it to lay down the power, but ultimately that means reducing the power to the front wheels. With the LSD being a truly mechanical setup, when one wheel is spinning, the hydraulically controlled LSD can lock up the diff 100% if needed to ensure that full power is given to the wheel that needs it, obviously resulting in higher speeds though the bends.
I think ultimately in a few years time, well see a lot of the GTI boys mapping their cars to 250-300bhp or even beyond, and all that through the front wheels. It's then that the LSD will come into it's own. TBH, never had one so won't miss it, but I think if a similar performance package was offered on the GTD for the same price (bigger brakes, LSD more power), I would probably have ticked it like the GTi boys, as I'd hate to regret the decision afterwards.