Thanks for all your answers
I'll check my savegames, but I'm quite sure I went far over *3 the total income.
In the stat view, on the income tab, my graph made a very steep increase which really looked more like * 10 than *3. Seems to me the income tab does not take upkeep into account ?
I went from 4 or 5k to over 30k income...
As for why I'd need more money, well, you can't ever have too much
Well that's wrong because the spell ruined the game !! But truthfully yes a bit more money would have been usefull, even in mid-game. I was in a game with *3 magic scale, playing MA CTIS and yes to propagate my miasma I had DOM 10, playing with an emprisoned pretender (I mostly play SP, that shows !).
So the dominion in my provinces was very high, with miasma and some order scale. Even then I can't buy a mage at each of my fortress (Yes, I'm SP player, I probably buy too many temples, labs and fortress). So a +30% increase would have been already nice. a +300% is too much. A *10 is game killing.
To double production is IHMO balanced, as you still need gold to buy troops. It's a nice way to produce more sacred at capital if you're production limited and a nice way to produce heavies, closer to front.
You probably won't double your recruitment, or not for long.
To double income is IHMO inbalanced, as it has a more global and profund effect, effectively you can double your army (upkeep).
In my view, the spell should be toned down so as to be still usable...