Managed to further refine the framerate effects a few hours later. This Stopper Watch powerup here will temporarily slow down time when you collect it. Pretty neat! Of course I didn't just code all of this framerate stuff just for one powerup. You'll be able to play the entire game at different framerates, and the engine basically motion smooths the positions of things between the actual game logic frames.
For future games such as the VERY long in development Super Freaks 4ever, I'd also like to include time slowing abilities for the players to use. This powerup here is a stepping stone in that direction.