I can access my uv channels (uv, uv2, uv3, etc.) as either a vector2 or vector3 in Karma CPU, but only as a vector3 in Karma XPU. Is there any plans to sync up the behaviour in CPU and XPU? Having to output the node as vector2 is causing some compatibility issues with other renderers.

That is odd - I'm unable to reproduce it on my end (both vector3 and vector2 primvars can be read with either vector2 and vector3 signature on the geomprop VOP).
Can you please file a bug with an example scene? Thanks.