Noel:
You can change the average driving speeds used by MapSource on different road classes when MapSource calculates your route, but you cannot change the average driving speeds used by the GPSR itself when the GPSR calculates your route.
If you change the average driving speeds in the MapSource application, then create a route and download that route to your GPSR, the route will be preserved (meaning, it will remain as-is), providing that you have the same cartography loaded on both your PC and your GPSR. If you have different cartography versions (e.g. Version 7 on your computer, Version 8 NT on your GPSR), the route will be recalculated, and your GPSR will use its default internal speeds to do the recalculation.
You don't have to do any map reloading - the 'unique route' you create is sent to the GPSR when you download the route, not the maps.
Michael