Ah, good. This is what I was asking for.
Before I go into detail, let me make clear that the penalty should be equal to the number of attacks, not the number of extra attacks. The way I wrote it in the master list is bad. I changed it in my own mind, but I never got around to editing the power here. Sorry about that.
Also: Fate Points must be spent to boost the individual attacks of a Rapid Strike. One invocation doesn't boost them all. Also, you have to roll individually for each attack.
Anyway, I already know that Rapid Strike gets better in comparison to spray attacks as your skill rises. At Great skill, you'd be a fool to use Rapid Strike. At Fantastic skill they're about even. At Legendary skill, Rapid Strike is clearly better. But even at Legendary, it's not such a huge deal. You get 2 attacks at Superb with a spray attack and 2 attacks at Fantastic with Rapid Strike.
If accuracy goes much past Legendary, Rapid Strike is borked. Fortunately, I don't think it does. (Ignore evokers, they can't use this power.)
Of course, all this leaves unanswered the question of whether single-target spray attacks are a reasonable effect. I'd allow them if a player spent a refresh on a stunt granting the ability, personally. Here's why:
As BumblingBear showed, normal attacks are better than spray attacks when the weapon rating doesn't exceed the armour rating by much and the attack skill doesn't exceed the defence skill by much. Spray attacks become better when you are superior to your opponent. But if that's the case, you don't need any extra power to win. And if your opponent has any reasonable chance of dodging, then the spray attack is lousy.
Also, 2 9 stress hits is not as good as 1 18 stress hit.
Anyway, that's my reasoning.