I'm not that experienced in custom firmware but while the digic 4 processor remains intact, there were actually many new hardware changes to the 550D which may lead to possibly different firmware. Apparently the reason Canon opted for 20 fps 1080p and 30fps 720p is that the sensor itself couldn't handle the bandwidth required for and higher fps, but I don't know for sure. There is a new sensor in the 550d, and higher resolution (though marginally) lcd screen along with button additions.
I, having a 500d myself am also quite disappointed about this. Though, this is exactly why I bought a Canon since chdk exists, but I may be leaning towards selling the t1i and buying the t2i.