Best would be to check in the shop, if the 8.5 boot fits the M binding you intend to get.
I'm between the M/L binding sizes as well, (my shoes are usually size 8.) I've combined size 8 Burton and size 8.5 Ride boots, both with size M Burton bindings.
I rather have a binding at the smaller end than at the edge of being too big. Too big length wise means you loose leverage, ability to use the gas pedal, and in terms of wigth you may slip sideways.
Ok, I was originally leaning towards the L but after thinking about it I'm thinking try the M first. I can exchange for cost of return shipping so no big deal. I'm guessing they factor in a decent amount of play on either end to make sure they accommodate different brands and models.
I know my sons smalls technically start at a 4 but his 3s fit them no problem. I think they punch way more strap adjustment holes than the size range they specify.
And it seems it'll be easier to tell if it is too small as opposed to whether it's so large it may not be ideal.