For reference I did some testing of a level 6 mana regen troop (5%).
5% means 5% apparently: a slow mana hero went from 12 tiles to charge by default, to about 11.5ish (hard to say since doing graphical interpretation, looked at 11 tiles to be aroune 11.5-11.6 filled).
If we presume that is correct, to go from slow (12) to average (10) we’d need 17% mana regen speed; therein lies a non-trivial problem.
This picture was taken from Odie about a week ago, notice the absurdly high level, and then note the mana regen stat:
Ultimately it’s really not worth investing in imo if it can’t even take a slow to average at that level which I will probably never ever get to for my troops. Might have more use on defense, maybe, but given the expense to level troops, if I had a 4* crit I’d never ever wind up getting to the 4* mana regen one… and if I didn’t, still hard to shelve a good 3* for this one: crit is good on both attack and defense, whereas mana regen is effectively a wasted stat on offense.