Does your car not have any bumpstops on it? Or do you mean you just haven't swapped out or trimmed down the ones that came on it? Either way, I don't think the bumpstops are an issue for your problem. I guess in theory you could put much taller rear springs on to jack the rear of the car up enough to make it hard for the tires to come in contact with the fender lip but realistically you need narrower tires or wheels with a greater offset.