My S1 RST is lowered and even when one of the rear shocks was blown with positive camber on the rear wheels they never caught the arches. Now that the car is on Nankang NS-2Rs (a track based compound, thus the tyre is wider like other track biased tyres like the Toyo R1Rs, Yoko A008Rs, Federal 595RSRs) I did have an issue up front for different reasons but the rears despite running closer to the arch are still fine. The rear does now have a little more negative camber after some fault finding which would help matters, but despite the extra width the tyres do not catch.

What shocks have you fitted, and how did you install them (I.e, did you have the spring pan jacked up when you fitted the shocks into the place or the floor)? What shape are the plastic arches in and are they fitted well? The top damper hole where it goes onto the stub axles of my Ledas are elongated, meaning that when they were fitted via having a jack under the spring pan, they always went to positive camber ; the builder of the car made this mistake and I almost did.

It is unlikely to be the issue but it is possible. Without seeing the car in the flesh and checking things I am struggling to think of what else it could be bar:

severe toeing out making the wheels catch somehow (I doubt this strongly!)
worn bushes causing the wheel to be sticking out
stub axle holes in the dampers drilled out incorrectly

Here is how mine sits and how wide the tyres are. As said before I changed the front, the fronts almost always caught the arches upon going over a speed bump.