Not sure where the much bigger power to weight ratio is coming from Fraser?
Brera 2.4 JTDM 1600kg 200bhp = 125bhp/tonne (2.0T-spark 156 was 128 bhp/tonne btw..)
Brera 2.2 JTS 1470kg 182bhp = 124/tonne
Alfa 159 1.9 JTDm 1525kg 148bhp = 97bhp/tonne
Alfa GT 1.9 JTDM Cloverleaf 1365Kg 170bhp bhp/tonne = 125
Bearing in mind that Alfas "official weight" claims for the original Brera were found to be 100Kg out by Autocar, the 125 bhp/tonne is probably closer to 118bhp in reality..
BTW I wasn't really saying a 1.9 GT must have a Q2 or it would be useless or anything..
The 1.9 was pretty much in the middle of the GT/156 range powerwise (2.4JTD, 2.5v6, 3.2v6 ahead of it)
170bhp is a common enough output for a 1.9 turbo diesel and the cars should handle it well enough.
Even at that relatively sensible end of the spectrum diffs are still useful if you expect the car to be driven very enthusiastically - the harder you expect the car to be cornered the more useful the diff is (since the outside wheel gets unloaded more).
At 177bhp/tonne an LSD certainly should have been fitted as standard to the GTA / GT 3.2 though.
So why would you need it or get benefit from it on a 150 to 170bhp 147 or GT when the power to weight ratio is much smaller than the larger engined 159/Breras?
You probably dont need a real diff in the 159/Brera variants anyway, since the bhp/tonne is relatively modest.
I'm sure the Q4 is a very nice drive, but you normally would need much more power to warrant it.