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Still a pretty respectable image JJJ - Luminance wise I'd be pretty happy with that myself, save for some streaks here and there.
Have you tried getting rid of the color noise by applying a big Gaussian blur to the color information (use layers for this)? It usually does wonders for chromatic noise.
I'm not familiar with the 550D, but I know that on some other cameras shooting in JPEG doesn't do you any favors when it comes to astrophotography. On these cameras shooting in JPEG will put it through all sorts of on-board image processing (including hot and dead pixel removal as determined in the factory) filtering and sharpening to make an image look 'pretty'. Add to that that these image processing steps need to be quick & dirty (because people expect to see the finished result on the screen virtually instantly) and things become sub optimal very quickly (not to mention compression artifacts and the conversion of your 10 or 12-bit source to 8-bit for JPEG).
I also know that, on these cameras, if you take dark frames in JPEG, chances are that any hot and dead pixels are already (sub optimally) removed from your darks. And if they are still present, JPEG is quite bad at encoding sharp spikes, resulting in artifacts (and corruption) around a hot or dead pixel.
Long exposures in JPEG mode in some cameras also do their own dark frame subtraction, so subtracting dark frames *again* will cause a lot of grief, probably similar to what you're experiencing now. You can tell if this is the case if after an exposure of a specific duration the camera takes an equal amount of time to 'process' the image. In reality this 'processing' is the camera shooting a dark frame.
It all depends on whether your camera allows you to turn all this off for JPEG processing (my hunch is that it doesn't). If it doesn't then shooting in JPEG is usually a bad idea!
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