I am having the same issue with the Tresana. In my typical AE size, it has plentiful toe box space but leaves me with a slipping heel, even with a very tight lace-up (leaving almost no gap on the laces). I'm hoping sizing down will improve the fit, otherwise i don't see myself getting this model. Regarding bumpers and pads, I had a heel slip problem with my Moras, but a tongue pad did the trick. If you've never used a heel bumper, it is going to jam your toes further up the toe box, to the point that it might be uncomfortable. Aside from the fact that the bumper might aggravate your heels. The tongue pad though, pushes your foot back, keeping your toe box the same while improving your heel fit. Might be an option, but i would try to size down before making that decision.
Another option would be taking an awl to the strap and poking another hole in it. I'm reluctant to modify a shoe with no guarantee that it will work though.