Am I misremembering about a speed-limiting circuit in the Rad-spec motor? I seem to recall discussion about how even a higher-voltage battery won't make them spin faster, like the Bafang-branded motors do. I'm happy with 25mph on my MiniST, so have never pursued this aspect very far.
I have some experience with these motors. There is no speed limiting circuit but loaded speed (aka real topspeed on road) is combination of motor phase input voltage, motor KV and motor phase input current.
Increased voltage alone does not nesessarily increase topspeed alot of current is limiting.
Motor no load speed increased very close to linear with increasing voltage.
There is reason why topspeed and no load speed varies with individual "Rad motors". Rad has used motors with some different KVs. This is indicated by motor turn count number. As a result using controller and display without speed limit hard code max (40km/h) no load speed can be seen to vary.