Doc,
Thr ATB offers little over an open diff - pointless with the performance you now have, you want to be able to fully exploit it on tack and for that matter safely on the road too - so forget that as in term of bang for your buck its not a cost effective option.
The Drexler is the other end overly expensive, but a great unit.
If you don't mind rebuilding your LSD every other year and have limited performance then rebuild your Titan - again £800 plus every other year doesn't sound a great deal
Tracsport - designed for the purpose, tried, tested and proven before it was released for general sale, superbly engineered and offered a a sensible price.
Send your final drive to Steve at SPC in Redditch,