If other lenses work fine on the camera, only the new Sigma lens gives problems and then it's only in standard focusing mode... the lens is out of adjustment.
It's not surprising that Live View and standard viewfinder-based focusing give different results... That happens because they are two completely different methods of focusing. Live View uses sensor-based, contrast detection method, and is generally the most accurate, but slower. The AF points seen in the viewfinder use an array of separate AF sensors, in pairs, for phase detection focus, which is the fastest but can get out of adjustment.
In fact, switching back and forth between the two types of focus is a technique used to fine tune the focus on some camera models.... Those cameras that have the Microadjust Focus feature.
Your 550D doesn't have that feature, so all you can do is get the Sigma USB dock and directly adjust the lens itself. Or play Sigma Russian Roulette, swapping lenses until you get one that happens to be adjusted correctly for your particular camera. Of course you need a cooperative retailer to be able to do that.
Alternatively, you could send the lens (and your camera, perhaps) to Sigma and have them calibrate the lens for you under warranty.
You do need to test using a nice, contrasty target in good light... and testing is usually best done with a large aperture so that the resulting shallow depth of field rendered in the image more clearly shows any error, one way or another. However, if you set a smaller aperture it will not effect focusing with modern lenses. The aperture of your lens is maintained wide open during focusing and is only stopped down to the smaller aperture very briefly at the very instant of exposure.
One thing... do you have a filter on the lens (for "protection" or whatever purpose)? If so, be sure to remove it before running your focus tests. Filters can cause focus errors.
