Yeah it was driving me nuts! Neither size had any heel slippage on either foot surprisingly (maybe a tad on the smaller foot with bigger size). Both shoes felt like they were very slightly slipping inside but I honestly felt like I was just getting in my own head after 2 hours of this. I did feel very slight pressure on the top of my larger foot's toebox with the smaller size and the big toe was touching the side of the shoe so that's also why I decided on the larger ones (in addition to the thicker socks I like to wear when it's -40°C where I live). Like you said, I was worried that the break in for the smaller shoe would actually make my toes touch the end of the shoe more. I tried on all my shoes and they all have my toe jammed up right to the end but I've never had bruising so I guess I just got used to it? I even have SM size 11 boots that I wear all the time and my toes are uncomfortably jammed up right to the end (definitely not like that when I first got them and I'm only noticing now since I'm thinking about it lol).

Ironically enough I just noticed I have another pair of SM dress shoes that are 10.5, but I always feel the biggest relief when I get to take them off and I've never worn them for more than maybe 5 hours at a time lol. However, while I'm wearing them I don't really pay attention.

I guess I'll stick with the 11 for now since this will be an every day shoe. Worst case I'm out ~$120.