Well, if the 5% figure simply comes from a change in settings.txt, making the max maintenance reduction 15%, then you should be OK.
If the change in in the hardcode, like the part about not going below zero (even with +100% to maintenance reduction), then you're gonna pay 5%.
I doubt that SE4 will choke on your savegame, but you could always just go back to v1.35 if it does.
I'm going to finish at least one game at v1.35, dangit! So has anyone using the Devnullmod upgraded to v1.41 and played a v1.35 savegame yet? any errors?