Thanks for the advice guys
Re the diff options: The ball diff can be tightened by 1/4 to 1/2 - I'm not sure though that's a good idea with steel balls. It's noted though, I'll try that.
I also want to try a gear diff - sooner or later. It would be interesting to see what the different construction does to the forward traction - and if with the thick oils you can find better forward traction without making the car understeer too much.
As for the rear tires: there are so many things to try out - I could use stiff inserts, completely rigid inserts, glue the majority of the width straight to the rim and use only a small ring of the actual tire as a contact patch... In the end though, it's also about finding solutions that are easy to use as well as within the rules. I think playing with inserts and the tiretread would be the best options.
Lastly, the wing. Now I know it jumps stable (with this wing), I can start looking at the affects of running (and flying) it with less or no rear downforce