interesting
and curious - why RT56 rotors on a GRX bike ? - I believe you can use the better rotors (RT6x etc)
(for my one bike - RT56 rotors are installed due to the calipers ... low end M396 or something like that)
The RT56 rotors came with the bike. The groupset is GRX800, though I'm pretty sure the brake calipers are actually GRX400. Shimano's GRX product page does say that RT64 rotors are recommended for the GRX400 caliper. They recommend MT-800 (XT) for the GRX800 caliper. The pad recommendations for both are the same (
L03A-Resin or K03S-Resin) so I'm not sure what difference it would make. I'm running the L30A Resin pads (the ones with the fins).