my coldest ride is also -15f, an 80 mile loop around Lake Winnipesaukee, something I learned the hard way, after swiping your card at a gas pump, put your gloves back on before you grab the pump handle, no matter how much heat you have, you won't warm your hands, luckily, I only had 3 miles home.
I tried going out at -25f once, but by the time I got the bike (SV650) started, it had warmed up to zero.
I did once go for a spin up to the corner and back (half mile each way) at -25, I had been making some mods in my shed, just wearing sweats, yes, I was stupid, no helmet, no gloves, 0-80mph then stop, turn around and return, I was shivering so much at the corner, I was lucky to make it back, How long was I in the cold, maybe 75-80 seconds but some of that was probably -100 wind chill..... yes, stupid
I've been out a lot at -5 to -10, I've been going out some without my jacket liner, only mt Gerbings gloves, wearing Carhartt Extreme Arctic bibs & jacket, and stayed comfortable, but sometimes the electric heat feels nice