The solar valves release when there is a vacuum in the line without any pump pressure. However, what cleancloths was describing is simply negative head not a vacuum. The pipe will still be pressurized throughout the whole run. Head is added with a pipe rise but then is subtracted with a pipe fall. Net net is zero head gain due to the fact they are on the roof (the panels and pipe runs will add head). You just need the high speed to fill the pipes but after that, low speed is sufficient. So after priming, the total head loss for solar is the same if they were on the roof or on the ground, assuming the same lengths of pipe.