15mm offset will work...sort of. Basically pushing both fronts and rears outboard 10mm further than stock. On the rear you're fine. No issues there but perhaps a little faster long-term bearing wear due to the longer lever, so-to-speak. Pushing the front wheels outboard will change steering geometry. If you "Google" scrub radius you'll get an idea about what I mean. Essentially your wheels will tend to move in an arc around a point rather than pivot on that point. I ran 13mm spacers on the front of my 245 for a while. It made for some interesting steering characteristics. I'm not sure how the different suspension geometry of the 122 would react to the change.

For the record, my 17" wheels are sitting at the stock offset position. 43mm offset wheels and an 18mm spacer built into the rotor hat.

One way to avoid pushing things outboard while still clearing brakes would be to go to a slightly larger wheel. A 16" wheel would likely work. Running a wider tire on it would certainly make it appear to have a wider track...
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Dale