Hi Ian! Thank you for your hard work on this. The least I can do on my part is give you a report of what I have found so far:
1.- Indeed there is a slight increase in FPS but there is a significant performance loss when using transparent layers at full screen size (example 320x 240 rain sprites)
2.- There is also a loss of performance when using the perspective or wave effect in the sprites (using these commands you lose up to almost half the FPS)
3.- Using higher resolution apparently it becomes more stable and fluid but you automatically sacrifice about half of the FPS (from 30 FPS to 15 FPS but with less instability) apparently something was gained in the same way.
4.- An error in the "Start Game" menu if you go up from here the engine restarts.
5.- Animated GIFs continue to play relatively slowly, even with a small speed increase.
Outside of this I see that there are enormous possibilities of getting this to work at 640x 480 since I notice less drop in frames than with 320x 240 which is more unstable even using the effects of scaling, waves, alpha, perspective, etc ...
PD: I think the friends of Team Senile must have the answer to the increase in performance at the graphic level since they achieved it with the "Intrepid IZZY" engine that runs at 60 FPS and at a resolution of 640x480.