Basically in the dry the standard fwd of the 155 is fine, i dare say it would even cope pretty well with the standard 190bhp of the Q4, however, in the wet, and on the twisty stuff you cannot beat the 'grale/Q4 system, slight rearward torque bias kills alot of the understeer in normal driving, especially if you sort out the front suspension. But thats another story. And in the wet, well, you just don't worry, drive as normal the car has so much traction that you really have to be trying to get it to spin anything.
You mention that it looks fragile, in normal use its fine, but if you are thinking that you are going to be able to do standing starts all day long in this you are right it will be fragile.
I only ever pull away sharpish when the road is wet, then the tyres will break traction before anything in the drivetrain breaks.
I have only spun the wheels pulling away once, and that was pulling out of a side road when the road was very wet and there were leaves all about. Oh and my car has over 300lbs ft of torque on a good day!!
I'll be sticking to my 4wd from here on in, but then again if they ever make a 156 GTA rwd I might have to rethink.