Assuming both ships have focus the Phantom takes 3.3 shots to kill from a 3 attack with focus ship, while it takes 8 or so when it is cloaked. It's way more than double durability.
That said, that makes the hard counter far far more clear. And once that focus is stripped the 4 defense dice are far less effective.
The counter comes in that a Phantom cannot afford to close with a Falcon due to what 4 attack dice at range 1 can do to it. That leaves it at further ranges, where it is in more firing arcs, and can't manage to avoid them nearly so much.
On a specific match up like that - agreed: the extra durability can more than double. Conversely, at range 1 the increase should be less than double. It depends on a lot of factors, especially how often the defender has focus. I assumed 50%, to use the same weighted averages as I was using on the rest of my Lanchester's MathWing thread, to be consistent.
It's reasonable to say in that situation the Phantom will Always have Focus(unless they evade which increases the durability further.)