Find it pretty hilarious that people frequently blame the WR's. A good majority of receievers would tell you it's pretty hard not to drop a ball when it rarely hits you in the numbers or if it's thrown slightly too early. Not to say they didn't have drops, which they did, but Martinez played a huge part in that as well. You missed the biggest possible reason for the increase in percentage: improved footwork, which improves timing, and also improves mechanics. So better mechanics and better timing more than anything will increase his completion percentage.
I find it pretty hilarious that you think a ball has to hit a receiver in the chest for him to catch it. The rule has always been "If you get two hands on it, there is no excuse for dropping it". Do you think that every pass hits Calvin Johnson or Larry Fitzgerald in the numbers? Those guys still make the catch, don't they? Both parties are equally responsible. You can't demand near perfection from a quarterback, and then justify receiver mediocrity on the same hand...
But I do agree that any increase in his comp % won't be because of a lack of drops. More than anything, I'd say that being in Beck's system for another year will be the difference.
I don't know, I've never regarded comp % as that meaningful of a stat...Nick Foles completed 70% last year at Arizona, and look how well his team did. I'd rather see Nebraska utilize more mid/range downfield throws and have a lower percentage than incessantly throw dump offs and have a good percentage.