I've done the full monty on both my 156's. Thicker anti roll bars rear, softer suspension front, strut bars, higher airpressure in the rear.. and indeed you can make the rear end lighter. But all it does is bring up the rear more easily. You will never ever get natural oversteer..its just not something you get from a FWD car. The best you can do is induce lift off oversteer, and then fight to hold it.. But this is pretty much an artform to master, and not very easy! Power sliding/drifting in a powerfull RWD car with front end mounted engine is much easier to hold then lift off oversteer in a FWD car. (At least in my own experience)
Sliding over all 4 wheels is basically understeer with a ligther rear end. I think i know exactly what you mean when you talk about the "oversteer" you experienced. The car is still sliding across all 4 wheels yes, but car is not going in the direction the front wheels are pointing. So it's still understeer or scrubbing.
It's also my experience, that once the tyres are properly warmed up, the car won't slide over all 4 wheels anymore, and reverts back to understeer mode. Plus it's only possible to do 4 wheel sliding in very open curves, and at relative high speeds. Try doing it in a hairpin, and you will get massive understeer if you turn in too hard with too much speed.
In any case understeer is a lot safer, so unless you really know what you are doing in a powerfull RWD drive car, then you will have more fun in a FWD car. Everytime i go to a track day, and there are Porsches in the line up, i always try to avoid them, unless they really know what they are doing. Most of these guys have no idea how to drive their car on the edge, and those Porsches are very unforgiving! They always end up spinnng their cars in the corners, causing flag waving of one sort or the other