Anyone with experience of 15-85 (and ideally 17-55mm, too)?

I've used a 15-85 with my Canon 60D for six years. Mine is a very good lens that my only real complaint about has been its slow aperture. You can get fairly shallow DOF with it but it's not a great lens for that. But it is a fantastic general lens - FF equiv. of 24-135 is great to have. But, just to be sure I recently rented a 17-55 for comparison. As you said, this is one copy against another, but I found that the lenses for all practical purposes were equal at equal apertures. The 17-55 has 2.8, and it's good wide open, but I had to stop it down to f4 to match the quality of the 15-85 at f4.

So, if the f2.8 (at slightly lower quality) is a high priority and you can live with the shorter tele range, it's an excellent choice. The 15-85 is slower, but I find it to be acceptably sharp even wide open - which is a good thing as I shoot it wide open 90% of the time. The 80D having better quality at higher ISO would work better with these lenses than the 60D.

I found both to balance well - the weight is about the same. The 15-85 is shorter, but the 17-55 is well balanced and I didn't notice the extra length. Never noticed any decentering, but my 15-85 does front focus a bit and the 60D doesn't have MFA. Probably should have sent it in to Canon but never did. Also never used a filter on it.

For lower light needs I do have the 24, 40, and 50 STMs which are also all good. My 50 is soft wide open but good by 2.2 and excellent by 2.8. The 40 is excellent wide open, with the 24 very good at 2.8 and excellent at f4. So, I've been doing your exact approach - 15-85 for versatility but a faster prime or two when needed.

My mistake was that in trying to get better quality I also rented a 6D  

That was enough better that I have now gotten a 6D, 24-70 L f4 IS, and for low light the 35 f2.0 USM IS - all at great Canon Refurb special sales recently. I already had the 70-200 L f4 IS, and now just need to get a faster telephoto prime in the 85 to 135 range.

Hope this helps.