I was thinking more about my suggestion to connect the wheel house to the new edge of the fender. I should clarify (I think) and say that the rear fender/wheel well is made up of two pieces welded together at the fender lip. When you cut the fender away, you will have to also cut away the wheel well to the same height. You will save a little weight, but not more than a couple of pounds (a couple pounds is a couple pounds!) on the whole car. And without sheetmetal connecting the metal panel edges, the unibody is compromised. The front is not really an issue as there is no inner panel. Aside from that, there should be no reason to have any metal sticking past the attachment area of the flare.
I'm not really happy with the look of the Steinmetz flares. I'd like something that was more the same height as the stock fender. Make the car look a little more muscular.