We are not familiar with that service, but I would caution against expecting too much precision out of any photogrammetric solution at this point. It is likely that the realtime value will be more than enough.
The points defined by the GCPs are simply used as anchor points where we try to minimize their error. Even the GCPs are not guaranteed to be right on the defined coordinates.
The image based reconstruction is the same whether or not GCPs are used. The GCPs are only used to align the rendered solution to real world coordinates. Not stretching or scaling is done based on the GCP definitions.