High temps sounds like a reasonable explanation. It doesn't surprise me that smaller pixels are more sensitive to heat (greater noise to signal ratio!) but I doubt it is specific to the 550D - it's just the laws of physics and thermodynamics! So I still think the 550D/60D will look pretty similar to any other DSLR sensor of a similar vintage and pixel size, for a given temperature.
For astro use, I think buying a cheaper secondhand 40D/450D with slightly larger pixels has to be a good option. 20D/350D is even cheaper again and has even larger pixels, but has some amp glow in one corner and doesn't have LiveView which for telephoto lenses makes a big difference (not so much for deep sky). And of course, larger pixels is why I love my 5DmkII so much!
Phil