Thanks Blueraven & Kais3r for your additional feedback.

As it seems the 245m wide tyre clearance to rear guards is not an issue, I will seriously consider going to 245/35 all round. Tyre wall height will also increase slightly with this combo, but hopefully not enough to feel noticeably softer.

Regarding the Haldex unit & effect of varying tyre diameters, it will need to allow additional slip through it's clutch pack hence possibly increasing oil contamination & filter blockage.