Re: 156 2.4 JTD - why can't I overtake ! ? !
A q2 diff is really good, I have one, it will cost a little extra in labour as the bearing need swapping, crown wheel removing etc, but they can be had for £230ish + VAT.
Chipping is well worth while, the 136 should go to 180bhp or so, rolling road proven figure that is, not some higher BS claim for a chip on your driveway!
The LSD goes hand in hand with a good suspension setup and 17" with 225 tyres.
If you don't intend to do the suspension or rim upgrade, then I woud think twice about the LSD.