I think the problem with Maul (and it's not really a problem, per se), is that his abilities are too good to warrant a permanent speed bump. Have you seen his basic at 7* fully geared? 5k damage non crit, offense up for 2 turns and 100% turn meter on a finishing blow, and deals double damage to jedi. Like bruh lol. Same with his aoe; 5k damage, and deals double to jedi.
I like the suggestion @Aero had though. Either that, or like a 2k bump in health at 7* maybe.