Ainston
Although I can't make any direct comparisons with the Galaxy, my experience of touring with the RST leads me to believe that a 90 mile day will more likely end up feeling like a 75 mile day. I don't think it will necessarily get you there any quicker than a Galaxy - as FDR and Stutho both suggest, the only way to work this out is to do the same timed loaded route on both bikes in similar conditions, probably 2/3 times if you want a scientific conclusion.
I guarantee you however that the RST will prove to be the more ergonomic and comfortable bike assuming you have it set up right, especially on hilly, mountainous terrain where the reliable smooth changing of the Rohloff really comes into an element of its own compared to derailleurs: my climbing capabilities have improved since owning the RST, despite now being the wrong side of 40.
Unless you're young and/or super-fit, 15-16 miles per hour strikes me as ambitious for loaded touring over 90 miles, (13/14 mph strikes me as more realistic?) especially if there are significant hills or poor roads en route. If there aren't, you might consider higher gearing.
Good luck with the tour, am interested to know where are you touring from/to?