You can actually run larger tires on the rear without flares
I have a set of 305x18 Hoosier autoX on CCW rims and Goodyear 315 x17 on Fikse's with no problems. If you want to go wider or taller than that most people are putting in internal "mini-tubs" this allows +335 widths. Mallett does a nice minitub mod.
The ultimate "flare" however would be the Specter Werks GTR. Not the Group 5 Mod but the GTR which increases the whole body width by 6 inches!!