That's correct, the high detail reflections are a new feature in the nightly build (previously selecting "high" would just do "low" and not tell you the difference). Based on the profiling data from your GPU, it's spending 35 ms in the deferred ambient pass, which is where the high detail reflections are computed, so that's definitely what's causing the slowness.
Even so, can you look a bit in the code to see what causes it to go so slow. DirectX games using deferred engines behave so much better. Race Driver Grid, NFS Shift/Hot pursuit 2010 have no framerate problems. As I said, this game baffles me since no matter the screen resolution(640x480 to 1280x720) it has the same framerate with max reflection detail. Anti-alias doesn't slow the game, neither maximum view distance, neither anisotropic texture filtering, just the reflection. Usually lowering to 640x480 in any recent game the fps boost is enormous. This game doesn't care about anything else except the reflections. What makes Vdrift so different?
VDrift already uses a deferred rendering engine. What makes the high detail reflections so slow is that they're extremely shader intensive, they're using the same technique as Crysis 2's DX11 ultra mode realtime local reflections.