The first step is to identify (actually write it down) reasons why the current camera body is not allowing you to capture the images you want. In other words what EXACTLY is wrong with the T2i.
Are yu sure the problem is the body and not the lens. Perhaps what you really need is a fast f/1.4 lens. Possibly the problem is the way you are post processing the images. Are you subtracting a dark frame? You are shooting RAW format, I assume.
It is going to be hard to say what to get without knowing why the current SLR body is not working for you.
I am not sure what real value a fast lens would have when the sweet spot for typical fast lenses (at infinity) is at least stopping down at least 2-3 stops.
I would imagine (since I really don't do this type of photography now) that for digital needs the following would be worth investigating when getting the camera and lens(es).
1) Useful ISO range (which could be less than what is advertised as full range).
2) Ability to shoot exposures lasting more than 1 second and some up to 30 seconds.
3) Possible tripod tracking system akin to telescope usage
4) Appropriate lenses that are not only by focal length but noted for working well set at infinity or near infinity and possibly apochromatic or achromatic corrections.
5) Selection of best post processing software and filters/plug ins for handling RAW files
6) Understanding colour correction for b/w output based on subject matter (in film some filters might be used to enhance certain parts of the spectrum or diminish others to achieve a clearer, brighter and often sharper subject rendering).
7) Investigating various sites or books on night sky photography and the techniques used (both film and digital).
8) Best methods for dealing with inherent noise in low light photography without sacrificing too much 'clarity' of subject matter within the image.
I know that the above really is directed to the original poster but certainly it may be useful to others here.