As far as I was aware AMDGPU is used by default on most if not all distros
I really don’t think that’s the case, assuming you’re talking about AMDVLK (amdgpu is the kernel module used by all three Vulkan drivers - RADV, AMDVLK and the Vulkan driver from AMDGPU-PRO). Ubuntu and Fedora definitely default to RADV, and Arch Wiki recommends RADV unless you need something from the other drivers.
I noticed a performance increase after forcing RADV on NixOS so not really sure.
NixOS seems to default to RADV according to their Wiki. If this was a few years ago then maybe you might be confusing it with the ACO shader compiler for RADV? That brought a significant performance increase and eventually became the default in RADV. I remember using custom Mesa (the project that develops open source graphics drivers, like RADV and radeonsi) builds to massively reduce stuttering in DirectX games.
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