I've been assuming you get get one more missile each time you heighten (+1 at level 3, +2 at level 5, etc.), BUT I just re-read the spell and now I'm wondering if I have been wrong about this (plus it wouldn't make sense given how the spell works).
The spell says:
Heightened (+2) You shoot one additional missile with each action you spend.
That sounds like you can fire up to 6 missiles when heightened to level 3 (2 per action spent), or up to 9 missiles when heightened to level 5 (3 per action spent). I.e. one additional missile per action (normally you get one per action spent casting).
So am I correct about getting 2 per action (1 + 1 additional) when heightened to level 3?
I just wanted to confirm with everyone that I'm not off-base here.