All lenses and cameras are going to struggle more or less to focus in low light.
Some cameras are better: 5DII and especially 5DIII (it appears), for example.
Lens can help, too... an f2 or f1.4 prime allows in one or two stops more light during focusing than an f2.8 zoom, so the camera's AF system would have more to work with in low light situations.
A USM lens generally will be faster and more accurate focusing than a non-USM lens, too (Sigma's HSM and Tamron's USD that each offer on some lenses are roughly equivalent to Canon's USM), though there's a limit to how much the drive type might help in low light situations. Your particular Tamron lens is not USD, so don't expect it to be as fast, accurate or quiet as, say, a Canon 17-55 USM lens.
Comparatively, the 5DII and 550D AF systems aren't all that different. Both have 9 points and in both only the center AF point is cross-type. More about this below.
I don't have a 550D so can't really compare directly, but Canon rates the 550D/T2i's AF system to -0.5 EV, same as my 7D cameras. However, they rate the 5DII the same, though I know from experience that it's actually still able to focus (pretty slowly) about -1 or -1.5 EV, a little lower light than the 7D. The 5D Mark III is rated to -2 EV and most people find the new 61 point AF very usable in low light, so even that might be a conservative rating. At any rate, it seems the best Canon yet, focusing in low light. (1D series cameras are also quite good in low light. But I'm guessing that since you've ruled out 5DIII based on price, you are unlikely to want to spend even more on one of the 1D models. So I'm not trying to compare them here.)
However, you also can help the AF system. Your camera has "AF-assist" feature, which is done with the built-in flash firing short bursts to illuminate the subject and give the AF system something to lock onto. You can set this to work with Custom Function 7 (Page 194 in your manual. Note: default is "enabled", so unless you've changed the setting it should work if you just pop up the built-in flash... other settings turn it off or only allow it to work with an external flash or only as an IR beam from an external flash, if it's equipped with one). If you don't want the flash to fire, too, go into the camera's menu and under the first tab scroll down to "Flash Control". Select that and the first item you'll see is "Flash Firing". That's normally set to "Enable", change it to "Disable" and that will prevent either the built in flash or any attached external flash from firing, but still allow them to serve other purposes such as AF-Assist. (See page 147 of your manual.)
Less intrusive than that is the IR "AF-assist" grid that many Canon accessory flashes can provide. Perhaps some third party flashes offer it, too. You don't need to use the flash itself, if you don't want to do so. Simply set it up only as an AF-assist. Or, if you don't want to mount a flash, you can use an ST-E2 or ST-E3 wireless flash module mounted in the camera's hotshoe. These are a lot smaller than a flash and also can provide the IR "focus assist" grid.
You also will have more luck if you limit to the center AF point on your camera. It's a more sensitive "cross type".... while all the others are plain, single axis. The 5DII has similar layout: 9 AF points with only the center one as cross-type (though it actually has 6 hidden expansion points, but they only work in AI Servo and are also plain, single axis type). 7D has 19 AF points, all of them the more sensitive cross-type, though the center one is still more sensitive. 5DIII has 61 AF points, up to 41 of which act as cross-type.
