Whether I can lose the extra width via raising the car, or by arch rolling, or by camber.
Cannot go any smaller sadly, not even by a single mm. 20mm spacer is nothing, I'm on 30mm as it is!
Agreed, but I don't want to move to 19" wheels as it means another outlay on tyres as well as adding weight, even over the spacers.
Buy M6 wheels then, they'll be lighter than the Rotas and throw the tyres on eBay