I've added some comparison photos between the GRX400 and Ultegra RX800 rear derailleurs mounted on my 10speed 11-36t XT cassette and 46:30t crankset (using GRX400 10 speed shifters). On a side note, I'm using a GRX400 FD and it works perfectly with road cranks with plenty of H/L screw adjustment leverage. No need for a road specific FD, while still capable of handling GRX 2.5mm offset cranks. From what I'm thinking, the Ultegra RX800 is at it's upper threshold for optimal shifting. Any bigger of a cassette and it won't have that fast, roadie-like shifting.
Though if I decide I want more low gearing I think I would go back to the GRX400 RD due to the long cage design. I've been really curious about the new Microshift Sword 10 speed 11-38t cassette that's starting to trickle onto the market and I wonder if the GRX400 could handle that cassette while maintaining
crisp shift performance? Also looking at the Shimano cassette weight and price of the current 12 speed 11-36t cassettes, I somewhat prefer 10 speed. The 10 speed 11-36t Deore XT cassette is cheaper and weighs less. I really don't notice much cadence difference between 2x10 vs 2x11.
