I guess your camera is an Ixus220/Elph 300hs? (not an ixus300?) It's a good idea to spell it out, because Canon uses many different, easily confused designations.
In this case though, it doesn't matter, because CHDK currently does not have the ability to control these things on any camera. Determining whether it is possible would require someone to do a lot of reverse engineering work.
In general, I would expect if the hardware were capable of doing significantly better Canon would offer it as a standard feature. E.g. if they could do 240 FPS as 640x480 rather than 320x240 without any hardware changes, it would be a significant selling point. I doubt this was a software limit meant to protect higher-end models, because AFAIK no P&S released around that time had much better capability. Plus it would mean they significantly overspeced the hardware.
