A few years back I started Playdeads QA efforts. That has resulted in a full time QA guy and a part time guy who helps out.
Back when I started I would have said that that was enough. But then I went through my first certification process! That was X360, which was actually peace of cake compared to the later PS3 certification(and PS Vita which I did almost alone)...
We found at Playdead, that if we concentrate on making sure the game runs well and bug free, then we use 3rd party QA to verify all the stuff we don't like to spend resioucres getting good at. Being "certification requirements". So what we do is that we keep the game rather bug free all the way until we're close to a serious release candidate and than we do a first pass with external QA. Who'll return to us with a lot of platform specific bugs and of course some more "grind" based bugs from the game.
This means that we now have a solid list of eveything that the game needs to "be able" to do to pass certification. At this without spending so much time reading up on it.
Then we do 2-4 extra passes with external QA.
So yeah we've had good experience with using external QA(https://www.vmc.com/games/
), but be ware that they really wanna sell you a huge package. So a good idea here is to be super critical on what actually is needed. When we first approached VMC they wanted to throw 5000 test cases at LIMBO. And I was like WHAT!!?
Then you find out that 2000 test cases if for online multiplayer, and 1000 is for dance-mat support(joking)... And then you end up with a nice set of tests that does the thing.
I can't see how any "decent" size developers can ship a game without help from external QA. But then again there's a lot of buggy game out there, and they have to come from somewhere...