It's hard to say without being there, but maybe the reason for having a little trouble with the cutoff wheel is how you are using it. You should do a "plunge" cut and sink the wheel about 1/4" - 1/2" into the sheet metal near the start of the cut and move the wheel forward in front of you. Keep a firm grip on the grinder.
With the wheel sunk into the cut, it keeps the cut in a fairly straight line. You can steer the wheel a little to keep it going straight. If you run the wheel just touching the surface until it cuts through will result in a cut that isn't straight and wider than the wheel.
If you do it right, the cut should be really clean and straight and just the width of the cutoff wheel. When you get the hang of it, a quick zip and it's done. There is a guide available for an angle grinder that allows you to set the depth of the cut.
