offset shackles (how big a tire)

Can't see where you would get any extra room with just the offset shackles, you would need to relocate the front hangers too.

That being said I have 15x8 rims with 4.5" of back spacing and 255/60 tires. A rim with about 4" of back spacing would have centered the rim better in the wheel well and a 275 tire would easily have fit.