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skinny-
with a grx mid cage you can run a 46 anyway
The 1x version (817) perhaps (though it's rated to 42T), however the 2x version (815) is rated to 34T max largest sprocket (officially by Shimano).
But apart from that, MTB RDs have significantly bigger capacities, which is especially relevant if you want to run a wide range cassette in combination with a double chainset.
Tijs
Though you can't mix and match road and MTB derailleurs, which is a real shame because a road FD and MTB RD would be great. I do that on my bike, only cable actuated.