Yes it would, but you have to consider other factors as well before you just go and add another video card. For one, what is your current power supply wattage and make/model? For two 550ti's in SLI, you will want at least a 600W power supply from a QUALITY manufacturer. If you don't have that, then that's what you have to upgrade first.
Secondly you have to consider heat and airflow. I don't know what case you have, but runing SLI will cause ALOT of heat from the cards, especially if they are close together. You need to have plenty of airflow going through the case to be able to get rid of that heat. If not, it might dmg your system if it gets too hot.
Aside from that, if you have those things covered, yes, you can run another 550ti and get a nice performance increase.
hmmm.. a quick google search says its a relatively good product, so I think you should be fine overall. If you do have any problems, though, like crappy performance in games or even worse performance in games than just using 1 card, the PSU is the first thing I would look into replacing, just cause there isn't alot of information out there about it. Otherwise, I think you are good to go sir. Enjoy your SLI!