I recently played around taking food shots and used the 7D, we had made sushi and it looked terrific on the plate. I used the 15-85 at wide angle, when at 3.5 and f4 aperture and with IS made more than sufficient difference that its a better choice than a prime without IS. Not like the food was running away.
When I used the 85 1.8 and 50 1.8 with aperture of around 2 to 2.8 (handheld), to keep up shutterspeed, the shallow depth of field was definitely tricky. Wide open the 50 is soft.
When taking photos in a cave recently I started with the 15-85 and changed to the 50 prime, but in hindsight would have had more consistent results with the 15-85 and its IS
I guess in low light it really requires a tripod, live focus and carefully placing your subject and framing it carefully.
As I don`t have a tripod and was getting hungry, priorities took over.
I don`t think its the 550D or the 15-85 that is the limit, its plenty sharp, there maybe small incremental improvements in upgrading lenses and body, but may I suggest that most of your improvements will come from improving your lighting. For artistic value the 85 1.8 would be a good addition if you don`t already have it.