8mm diameter (622 for the 700c vs 630 for the 27"), so definitely no problem with the frame. Some brakes can't quite make the extra reach, but most can. If your pads have 4mm to move down from where they are, you're good to go.

The only big issue is the bottom bracket/crank clearance. You're dropping it more than 4mm since 27x1 1/4" tires are bigger than 700cx23 or 25 or whatever. Usually it's no problem, but if your cranks are longer than 170 or you have wide pedals, it's worth getting a good idea of how far you can lean over the bike before you go ripping through any tight corners at 20mph

edit: sounds like you're aware of all that.