Factory diffs are few and far between, although because they're torsen the drift boys bin them.
$660 for a gearbox? Shit the price has gone up, last time I looked they were going for $350-400.
A decent LSD is worth $1000-1500, however, this is what causes axles to break.
Mazdaspeed do billet? axles to fix this though.
I'm running a single plate heavy duty clutch, seems to be holding up to the power ok, although when I re tune it may slip on the dyno.... I'll cross that bridge when I come to it.