Go to a 225/40 sidewall, or add stiffer springs?
no. that's the same width, it wont help anything. and as 08wrxme said, swaybars (or stiffer springs) will help keep the suspension from traveling too far on the medium bumps, but it'll still rub on the big ones.
your only choice if you dont want to rub at all is rolling the fenders. you may be able to get away with just folding up the inner lip, that'll gain you another ~1/2". if that's not enough you can pull the fenders with a baseball bat, or one of these like i did:
you could also camber in the rear wheels like the euro guys but at the expense of handling and tire wear. also you could get a more narrow tire size, again like the euro guys and again at the expense of handling. it'll also only mean that you wont rub till halfway down the sidewall, but it'll still happen.
easiest solution: camber.
best solution: rolling/pulling fender.