Finally got a new phone (S21 Ultra, SD888) to replace my aging S8+, and with it I've finally stepped into the world of high fps gaming 

I've been running RR3 in QHD+ (3200x1440) and adaptive refresh rate (phone adjusts rate based on content, from 10hz to 120hz in preset increments) and it's been pretty silky smooth at 110-120fps (I use Samsung's Perf Z Game Plugin to see fps in game). I finally got around to race NASCAR with its 40 car grid, which would noticeably drop frames on my old S8+ when many cars are in field of vision (e.g. start grid). The S21U ran it smoothly too, albeit at higher level of GPU utilization (60-80%). That got me curious to try some other settings.

First, I tested QHD+ at 60hz. As expected, GPU utilization rate dropped by ~30% while keeping fps near 60.
Next, I tried HD+ (1600x720) at 120hz. Game downloaded new assets and rebooted, presumably because game detects lower res screen and uses a different set of assets than it does with higher res screens. GPU rate didn't drop as much as 120->60hz did though, dropping only 15-20% compared with QHD+ at 120hz.
Finally, I tried FHD+ (2400x1080). Game didn't download new assets, and GPU rate was the same as at QHD+.

TL;DR
RR3 is capable of rendering at 120fps with a predictable rise in GPU utilization. Sucks up more battery, but boy does it look buttery smooth.
However, RR3 probably maxes out at 1080P (or maybe even 900P) in its internal rendering; running game in FHD+ vs. QHD+ made no difference in GPU utilization. Chances are, if RR3 ever supports QHD+ rendering at 120hz, it'll require some next gen GPU. And never mind those 240hz phones.