I don't know, on pg. 195 it says:
Every point of Magic invested in mana-based abilities grants the character
one point to use with Magic-based skills. For all other purposes,
including the determination of the maximum level for adept powers,
the character’s full Magic attribute is used
This make me feel that to determine overcasting you use your full magic attribute.....so in normal BC place, any spell cast at Force 6 or higher is overcasting for you.
With the Drain thing, everyone suffers from having a harder time resisting drain of spells....this doesn't depend on magic attribute...it's a separate thing. Full mages would also have the "double whammy" ...cuz a mage with Magic 5 and spellcasting 5 would now be casting at Magic 4 + Spellcasting 5...and on top of that they'd be having to soak more drain....so mystic adepts take the same "hit" as mages basically....they can just choose to take the hit to their magic attribute through their adept powers or their spellcasting. Sure pure Adepts take a single "hit" but they can't cast at all to begin with.
You chose to take the lose of 1 magic point and lose power with it. This means that your dicepool to cast spells is still Magic 2 + Spellcasting. If you had chosen to take away from the casting part....you'd be casting with Magic 1 + Spellcasting, but you'd still have your powers.