Heel strike is one thing I could spend design time on - what you see there is narrow enough in front that I didn't have any ... except when my heel turned in and I'd feel a nudge on it. The whole thing is pretty narrow, and I could cut more off since it's a tri-fold, but unless it becomes annoying I'm going to leave it as is.

I'm not as convinced that it has less shock in a bag on your back than tight in a pannier. My back will take up some shocks, but there could also be higher amplitude bounces generating bigger impulses. It makes me want to use a couple of accelerometers and compare.