How many miles were on the gear box before you had the Q2 diff?
I wouldn't think that the Q2 diff is the weak point, at least correctly installed. Mine Q2 has done around 10 tracks days running with > 50% extra torque over a standard GTA so I would expect no isues for a normal GTA for a very long time.
The gearbox itself had done 4 years worth of trackdays (> 30), again many of these with a modified GTA.
Before I had the Q2 diff I did have to replace my gearbox at around 38000 miles, but it didn't owe me anything for the large number of track days it had done with a modified GTA. BTW it gave up on normal road use, 5th and 6th gear went (still got me home, good lad
For a very long time 5th gear had made a slight rattle under low load at around 4000 rpm - I suspect something just wore away.
Get a recondition gearbox from Alfa themselves, they are inexpensive and, apparently in many cases turn out to be brand new..
You may want to consider dropping a new clutch in at the same time, the labour to do it afterwards is almost the same as fitting a new gearbox..
If you plan on doing many track days an uprated clutch might be an idea..