The bit I don't understand though is that if for example the front will not understeer in a wet corner till 50mph on RE040's all round, then if you uprate the rears only to RE050's are you saying that this causes the front to understeer at less than 50mph now even though it's still on the same tyres?
Logically no, but if the limit if grip for a given corner in the wet for the 040's is say 50MPH and the 050's is 60MPH, whereas at 55MPH before, you would first get understeer, followed by a slide as all 4 tyres lost adhesion, with a combo of 040/050 F/R, the rears would still grip at 55MPH, so rather than inducing a slide of all 4 tyres, you would get pure understeer, which is different handling to what you would expect. That is what will catch you out, not that it will or wont understeer, but that it will do something different to before
And yes Rob thats what I meant to say, thx for the correction