When I say 15% to 20%+ boost, that is typical FPS boost, not some special case. Each camera uses a lot of main thread CPU for setup and culling.
Unity 6 LTS update is the plan. Much of those U6 perf gains are for SRP (HDRP), which would require a full graphics system update (shaders, lights, postprocessing), so not anytime soon for 7days or maybe ever. We are doing HDRP in our next Unity game, so we are going through those issues now and they are huge.