Sounds like width isn't your problem, diameter is. I run 255 width tires up front with aggressive offsets and spacers and it doesn't rub on the edge of the wheel. I had some very slight rubbing during turns but now that the tires and/or the fender liner has worn, it's not an issue anymore. If you are only rubbing slightly on the front and/or back of the fender liner, I would not worry about it.
2008 IB E92 M3| BBS | KW | Arkym | Platte Forme A.G. | Active Autowerke | K&N | Fabspeed | Dinan | Evolve-R