How Aegis affects magi

I've had an alarming thought about how Aegis of the Hearth affects magi. I'm hoping that I'm delusional again, but I would like some reassurance either way.

Assuming that Aegis must penetrate Magic Resistance to have an effect (much as Wards do).

If an Aegis fails to penetrate a foreign magus's Parma Magica (which is Order of Hermes-brand Magic Resistance), is he affected? If he casts a spell, does it still suffer a penalty of half the Spell Level?

For example: a 25th level Aegis is cast with a penetration total of 20. If a foreign maga enters the Aegis with a Vim MR of 27, and thus the Aegis fails to penetrate, does she suffer a -13 penalty on her spell-casting, or no?

That's one of my main problems with requiring the Aegis to penetrate.
Appearantly, the idea is that the Aegis only have to penetrate against Might scores - spells are always and automatically affected.

EDIT: This thread might have something useful. No promises.