As you say the coil pack is most likely faulty (Common problem).

Tbh a misfire will have totally different symptoms to an air leak.

What you can do is remove each lead in turn, if there is no change then this is the faulty cylinder (It's said you shouldn't do this with modern coil pack systems) so you could remove one lead at a time then start the car and feel for the difference as Ford would say .

If you do it with the engine running be sure to use insulated pliers, else the current will end up going through you!

Martin