Do you have links to the "best of" photos you've already taken? Night games under lights common? Indoor in the winter? How cluttered are the backgrounds? Do you use your 55-250 out at 250 most of the time? Does acquiring new gear mean selling your current gear? What exactly is your budget? Are you selling prints, primarily online viewing, running in a local paper? If you're selling prints how large?
If you can truly anticipate the action, then more frames per second really only results in more hard drive space used up. And if you can anticipate it to be in single shot mode and fire off one frame, is more likely in my experience to have 8fps fire off 20 shots and hope the action fell into one of those fractions of a second.
In my experience just a 70-200 for soccer is a bit on the short side for more than half of soccer shots (it gives you about 25% or so of the field coverage); on a full frame camera this will result in an even shorter reach (technically a wider field of view at the same reach, but you'll have more space around the action). I can see 2.8 being an advantage for the background cleaning, but I wonder if the 2.8 is needed for the lighting conditions you're in if this is outdoors during the day. If it's at night or indoors than I can see 2.8 or faster being a minimum and budget limiting you to 200mm. Also if we're just looking at primarily web size image posting then to do some heavy cropping isn't that big a deal, if you've got parents buying large prints and the wall graphics of their children, then cropping isn't such a good idea and that gap needs to be made up in focal length. You can use teleconverts to get more reach out the 70-200s and Canon's latest TCs do quite well, but still not as good as lenses of the native focal length.
Before getting a new camera you need to look at getting out of the sport auto mode, the 7D flat doesn't have it, and you may find an improvement from being able to utilize all the other options in the camera that were previously inaccessible. I'd suggest a good starting point being Av with the aperture wide open. Then watch the shutter speed and do your best to keep it over 1/1250th and at that shutter speed go ahead and save your battery you can turn the IS off on the lens.
If it were just daytime, I'd say look at the 400 5.6L (for the faster AF) and the 100-400 4-5.6L (for the focal range flexibility) and keep your 55-250, then pick up a 7D or 7DmkII (if it ever comes out) this summer and then run the longer lens on the 7D and the shorter lens on the Rebel, put the camera with a longer lens on a mono pod and put a camera strap on it put the Rebel on something like a Black Rapid where it rests at your hip, when the action gets close the long lens and camera rests against your shoulder while you pick up the shorter focal length camera to get the action nearby.
If night photos are a significant consideration then I'd look to the 70-200 2.8s first and upgrade camera bodies as the funds become available (again 7D update may be approaching soon, but that's what they said last summer too, if it has a new sensor there may be even better noise performance). With nighttime images you're going to be working hard to battle image noise and keep the shutter speed up near 1/1250th, or even 1/500th. I'd look into shooting in RAW and the post processing (photoshop) noise reduction techniques there.
Also if you upgrade to a 7D, it takes CF cards so look at setting aside $300-500 to upgrade your memory cards because you'll be shooting a higher volume of images, in a larger file size, on a camera that can utilize higher write speeds. I went from 8GB on a 20MB/s SD card being enough on a Rebel in jpeg to 90GB not being enough, and anything under 800x cards really being too slow for shooting RAW on my 7D, and I only was taking 30% more images even with the 8fps. Also the 7D's auto focus does take a fair amount of learning to be able to maximize it, when you learn how to exploit it though the successes do go up quite a bit over other systems in the general price bracket. I mention this as it's something you don't want to be trying to learn just as the team heads for the playoffs.
I also wouldn't rule out the 60D and the alleged 70D that's also supposed to be released sometime soon. The 60D and the 7D have the same sensor, the 60 just doesn't have the same dedicated feature set as the 7D, I know this is primarily for sports so the expense is easier to justify, but if you're really budget tight it may result in a significant improvement from where you are at in terms of noise, and AF performance.