You are right there is roughly a 20% difference between what fpsVR show and what windows task manager shows, with task manager being the higher. For clarity, all of my CPU percentages below are from windows task manager. All of my frame rendering times are from fpsVR.
I started doing what you said and there was only a small jump in frame timing improvement between running at 50, 100, 150, 200 and even 300% resolution. My next thought is this has to be CPU bound.
I shutdown as many tasks as possible, chrome browser, items in the quick bar, etc. even the Advanced Frame Timing tool in the SteamVR settings, which I noticed uses roughly 15% CPU. That reduced my general CPU usage with SteamVR running to around 70% before running a game (it is usally 80-85%).
Then I started my reference game and slowly increased the resolution from 50% up to 300%. The CPU always stayed below 100%, and the CPU refresh rate well below 9ms, even when I was at 300% resolution! It was only really the GPU that went up each time and even that only went above 9ms (to 12ms) when I was at 300% resolution.
So for me anyway, this is most definitely a CPU bound issue, regardless of the resolution. In otherwords if I hit 100% CPU, even for a short time it will result in poor performance, which I guess is what you'd expect.
I think the only way this can be resolved is for Steam / HTC / whoever to improve the efficiency of SteamVR, which currently stands at 40%+ CPU on my fairly decent system, and the Compositor, which stands at 35%+ CPU, which is collectively between 75-80% CPU and that is before running a game, which will take upwards of 10-20%.
Until they improve those two items I will need to play with quite a lot of things shut down or build an even more expensive CPU beast (mine was pretty expensive as it is).