I have 18" JR15 wheels, currently running 9.5J all round, +40 fronts and +20 rears. The rears scrub a little on heavy cornering/bumps with an arch roll, but definitely liveable. The fronts fit the arches perfectly with no scrubbing, I wouldn't want to go any more aggressive at the front though as I'm pretty much at the limit of being under the arch. The main problem I have is because they are +40 I can only fit J Spec brakes as anything bigger would foul the back face of the wheels
Because of this, I'm changing the fronts to 8.5J ET+23 so I can fit K sport brakes underneath, and still hopefully be just underneath the arches.
Hope that helps!