Mana Leak is indeed even more powerful with a Rogue, assuming you can find a piety yielding glyph and Mystera soon enough. However, I'm less certain now that it needs any changes at the moment (except to make it stop triggering Mystera's anger, assuming that that's a bug and not a stealth feature). So far I've been unable to succeed with it in any of the vicious dungeons (without preparations other than scout altar). Actually, now that I think about it Creeplight is probably better suited to this strategy than just about any other dungeon - it's loaded with Shades who have very low HP even at high levels, doesn't have any enemies with a magical attack so you don't have to worry about angering Mystera that way (aside from the aforementioned as well as glyph conversion - although Anoobis does have a magical attack in his first form, killing him does not register as a "kill" for experience or divine wrath purposes), doesn't have anything with first strike or death protection (both of which are a major wrench in this strategy), and has lots of open space which translates to more tiles for regeneration (most of the vicious dungeons are tunnels, and often have their subdungeon inaccessible until the boss fight).
The changes that I think Mystera most urgently needs are one, to weight BLUDTOPOWA less heavily in piety gain, and two, to be more friendly to fireball casters and less abusable by CYDSTEP and HALPMEH users who don't care about monster magic resistance. (Although with the new changes to PISORF, I am interested in testing that as an alternative to fireballs for my Mystera-worshiping mages.) Do that and Mana Leak will probably be just about right as is... maybe.