The mana speed boost is exactly 2% faster mana generation, which alone, is absolutely worthless, as it won’t charge up in fewer tiles than before.
If you have a 4* mana troop you are using on that hero, there are breakpoints for those, depending on the hero’s mana speed, and for that hero, the breakpoint is one lower:
- Very Slow - Improves with a L1 mana troop and again at L23 I think, but no 4* hero has very slow mana anyway.
- Very Fast - Improves with a L11 mana troop
- Slow - improves with a L17 mana troop
- Average - improves with a L23 mana troop
- Fast - improves with a L29 mana troop
If the hero in question has fast mana (as you indicate), my initial thought is don’t bother: To get any improvement you need a L29 mana troop (nearly unachievable unless you spend a couple thousand dollars on troop summons) … or if you take this mana boost, you will get the effect with “only” a L23 mana troop (which typically can be achieved in a over a year of playing to get the feeder troops).
The place where this looks better is for an average mana hero – Getting a 4* troop to L17 can be done by farming in a ~year of gameplay farming, and add this boost to get the improvement; instead of charging in 10 tiles, the hero will charge in 9 tiles (three match-3’s)