I CAST BEARD
That would be such an awesome spell. And having the beard amplifies the power of your magic... it's just a really expensive spell with a time limit, one that is prohibited except in the most dire of situations... which leads to epic scenes where mages (of either gender) sprout massive beards, save the day, and shed at the end of their glorious rampage.
What if someone under the effect of Beard cast Beard?
If you were rich/well-supplied enough, you could achieve UNLIMITED POWAH due to exponential beard. Also killing everyone by smothering them in luscious beard hair, but that'd just be a side effect.
Since the spell has a time limit, and there is a limit to how much you can cast, at some point you'd reach a Critical Beard Density where it becomes impossible to cast any more Beards.
Well, the time limit only means that there's a limit to how high the Beard multiplier can go, provided that Beard doesn't also reduce casting time. The maximum Beardiplier possible would be equal to m^(d/c), where m is the base multiplier, c is the casting time, and d is the duration. This assumes that Beard also keeps casting time constant. If casting time is reduced by Beard, and the reduction pushes the ratio of d/c under 1, then the theoretical maximum power is infinite.
The main problem with this is that you'd experience a drop in power after casting a spell proportional to the casting time of the spell cast, due to the interruption of the delicate balance between casting Beard and the time limit. So, Beard would give you only a scarce few maximally powerful spells, with a decline in power after that point until equilibrium is again reached. Unless it is possible to cast more than one spell at the same time, ofc.
I may have over-thought this a
little.