The issue with SIM vs no-Sim in a cellular iPad is not only about connectivity. An installed SIM allows the GPS to launch the assisted, A, Part of the A-GPS and gives a much quicker first fix. This is even more pronounced when dropouts appear - if there is a SIM installed and you are in an area with mobile coverage, the position recovery is much quicker. Many times I only see the dropouts on my iPad cell only in retrospective, because recovery was so fast I did not noticed them in flight (BTW: this is another point towards internal GPS, as I think the external GPS boxes are non-A-GPS).