I have noticed that comparable nvidia cards appear to hold up better at maximum/ultra detail settings in Assetto Corsa, compared to AMD GPUs.
However, Radeon GPUs can achieve a 2X increase in frame rate by lowering certain graphics detail settings, with little to no apparent change to visual quality.
Please see my post here: Assetto Corsa R9 270 2GB performance optimisation which includes a link to slide by side video comparison of ultra vs optimal settings for an RX 580.
Given you have indicated you are running at low details... perhaps the switch from nvidia to AMD has brought a CPU bottleneck in play - i've heard that VR can be quite demanding on the CPU, and AMD cards can become limited by DX11 driver overhead whereas nvidia GPUs are able to make better use of DX11 multi-threading features to keep frame rates up.