The "big" control box uses slightly less electricity because it engages the start winding with the run winding on a Franklin. I can't speak about the Grundfos, I really don't see too many.
I do know from experience that you can use a Franklin box on a Grundfos pump. I would put in a 1-HP Franklin "big box" (they make one that will fit either 1 or 1.5 HP). It should work. The amps difference is only slight so I'm not sure if you're really gaining anything. Might be best to measure before/after wattage to see the difference.
Best of luck.