Just came across a mention of Hope RX4 calipers elsewhere and realised they do a Flat mount and a Post mount, while also being compatible with Shimano hydraulic systems... Could be a solution? 4 Piston so should be good, but think/expect they take different pads to Shimano so unsure how they directly compare when it comes to power on the road. Worth researching/reading up on how they compare. Of course you get colour options with Hope, so you could also switch to their coloured road rotors. Proper winter bike bling!