It doesn’t even have to be a pattern effect command, but I’d like to know how to do this.
Adjusting speed is too imprecise.
the BPM commands does not allow you to go over 255.
ReNoise starts a new song with a random BPM value. This is a nice “feature” which is somehow inspirative. It occurred to me once that a value higher than 255 was selected 
the only way I can imagine is to use note delay and note retrig to increase BPM locally, and pattern speed to increase speed in a wider zone, but as you said this is not what you may want, although I don’t understand what you mean by “is too imprecise”: if you want 300BPM at speed 6, just use 150BPM at speed 3, what should be imprecise in this?
I don’t really want 300 bpm. I’m actually working at 1/2 the displayed bpm.
I like the finer line resolution at spd 3 using twice the actual bpm.
eg. spd 3 360bpm I’m really using like it’s 180 bpm.
I meant imprecise as in speed being a divider or whatever and combination bpm/spd adjustments changing the effective line resolution.
I guess it might work for slowing down something I’ve already done at a non-effect command supported displayed bpm.
Then again maybe it would change how it sounds.
I don’t know. I’ll have to try it later.