What have drivers found, widening front tyres compared to rear - eg 215 F, 205 R - with a tad of neg camber will this alleviate roundabout type understeer without making it unstable on fast sweepers - ie in magazine speak 'more planted'

I don't think anyone has done this. I know of one car maker who has done this with the tyres and that is Audi on there latest sporty A3 (I think)

Did it with my trackday Alfa - 235\61\17 fronts with 205\62\17 reas - same compound Pirelli slicks......works OK but only if rear tire pressure is about 30 - 40% lower than fronts. With same front and rear tires is even better.....but my Alfa's rear end is considerably lighter than average(being trackday car only), and with stock Coupe things may be different.
IMO you will have better turn-in in low and middle fast turns, but you risk to much in faster and longer turns. But i might be wrong, it's only a guess......