Hello,
I would like to know how you read the use of performance magic with Brawl or Martial Abilities, i personnaly read it as such :
The magus Attacks and defend with 0 on is die and +3 Botches, then cast a spell normally (concentration roll).
I read it to say the magus may not attack at all (no more than any other magus casting a spell in a given round may attack in that same round). Even if by some miracle he connected, there would be no real strength in it: he is performing, not fighting.
I'm not too sure about defense, I think I would allow the magus to roll, except that no combat ability or weapon defense modifier would apply: a straight Quickness + die roll.
I'd allow it.
The book allows for a range of actions in (Performance) Magic, including Hunting, where the effects last for as long as the magus is hunting for something, following a track etc.
That means he's doing something which involves concentration and attention (hunting) while using his magic.
In order to cast the spell, he'd have to pause in a round of combat to do so. However, so long as combat continued, the spell would remain in effect.
Not sure its the most useful duration, but again, I'd allow it.
Edit: To be clear, the virtue of (Performance) Magic is that it allows continuation of an activity to maintain concentration for a caster. Its a new duration: Performance, which means concentration rolls are no longer necessary.
You must pause to cast, yes, but once cast you could attack as normal with no penalty. Same as music: there is no penalty to musical performance when using "Performance: Music Magic." Ditto hunt. Why would it be any different for combat?
I wouldn't allow attacking. There's no mention of why you need the roll, is it attack, defense, or use? Since nothing is said about penalizing Athletics or Music in combat, that roll has to be something else than using Performance Magic. Moreover, nowhere in RAW is it possible to do 2 actions on the same round, which removes attacking from the list. I conclude this can only be used for defense.
I am sure you can use your full Dodging if you accept the Concentration roll. Again, nothing is said about Athletics or Music, so you should be able to dodge. So using your full Brawl for Dodging requires a Concentration roll.
With that said, allowing your full Brawl without forcing a Concentration roll would make that Performance Magic much stronger than the others. OTOH, a Defense Total of near zero is suicidal and would make that option pointless. Even using a full Defense Total with a zero rolled should on average result in an Attack Advantage in the 8ish, I hope your Soak is above 12.
tl;dr Performance Magic (Brawl/Martial) should defend with all the bonuses. No attack, no Concentration roll.
I'd allow just treat Brawl as Sing or Hunt -- i.e. without a lot of the penalties other posters are suggesting. I do not think there are any game-balance issues at hand. After all, allowing a magus an extra punch or sword swing is probably worth no more in a fight than allowing him to bring along an extra grog. Plus, it exposes the magus to serious injury risks, and heavily limits the use of Performance magic outside combat.