I'm running 235/40/18s all round and they are wider than 7 1/2" wide too be exact they are 9 1/4"give and take a mill or so I'm using 20mm spacers all round as they are close to the dampers on the front with out them
255s 10 inch may be gettin tight I'm not sure what clearance you will have on your inner arch
you could consider having the arches rolled and flared a bit so there is clearance if they are going to rub
but it all seems a lot off messing about too me too have to fork out for just some slightly wider rear rims a merc 55 AMG uses that sort of width of tyre due to the the power output that the omega DOSNT have i don't think the cost will mirror the gain if there was one when you have lots of horses or use it on a track tricked out to the max i cant see the point that's only my opinion!!!!!