Emphasizing incline presses is a good idea. Grip width can't hurt either.
However, I think the crux of this is diet.
STP himself says that he's making strength, but not size, gains. He also notes that his diet is based on "good old home cooking." To me, that says, "I'm not eating tons of small meals and very high protein."
The solution is therefore a matter of diet first, exercise selection/performance second. STP, bro!, start eating more protein. Home cooking is fine--good even, since it's not had an adverse effect on your bodyfat--but you've GOT to pound down that protein every couple of hours. You know this to be true, deep down...just cultivate the same discipline you exercise (haha) inside of the gym, outside; every 2-3 hours, drink a protein shake or eat a bar. If you aren't gaining weight as your strength increases markedly with the 5x5, then you need to eat MORE.
Small, very frequent meals with lots of protein. If you're doing 200g/pro now, shoot for 350. If you're doing 300g, go for 400. As I've said before, consistency is the key.
While you're doing that, I also recommend you pay special attention to how you perform each pec movement. I used to like bench presses, but I agree with LS--inclines are generally superior. As Louden said, do NOT bounce the bar off your chest.
Also, I don't know how frequently you're doing the 5x5, but I dunno that you should necessarily change your primary movement every two weeks. That idea actually has some merits, but to put on some serious size, you'll have to make some pretty mad strength gains to boot...you can only do that if you're able to do a particular movement as frequent as possible.
That said, perhaps you'd benefit more from a somewhat different routine? DC training (read the training sticky) lets you hit pecs 3x every two weeks, and it's fantastic for strength and size gains.
Enough intelligent, experienced bodybuilders have touted 5x5 that I trust it's very worthwhile, so don't get me wrong...but since it sounds like pec growth is a priority for you, don't feel that you're stuck doing 5x5 if it's not yielding the growth you desire.