Fitted properly (with a little bit of fettling) there is no issue with rubbing on the tyre.
When I fitted them to my car and another TM members car, I took care to ensure they didn't rub as I felt it would be stretching the MoT exemption too far if the tyre rubbed on stays rather than the block.
Regarding loading on the chassis, my feeling is that the stay will deform before the chassis does.
I've also seen the SLR without them fitted and I was told why they weren't fitted but I'm blowed if I can remember
