Just because the effective top tube length is shorter than your old 58cm frame, resulting in the need for a different stem length, doesn't mean it isn't a 58cm frame. The frame size refers to the seat tube length, not the top tube length.
That being said, Lightspeed seems to use small, medium, large, etc. sizing, so you have to look at their charts to see the actual measurements. They say their large is a 56-58cm frame, so...
If you can get the fit right okay. If not, why mess with it?