Thought this may be handy for a few folks around here. I just realised after doing these tests that mymes of code to handle manual drawing for GMOSSE's stage 3 boss was a waste of time purely because of how this plays out in GM's order of events.
Any other tests or insights from GM users around here please do speak up!
I usually just use the h/v speed variables for stuff that doesn't need per-step control (eg bullets) or only needs it set once. If you're doing h or multidirectional scrolling affected by the player's movement or screen positioning, it's kinda out of the question unless you convert or nullify it in some way before the draw step.