Udet, going with the flow
a) I totally agree that one hit from almost anything would break the pressurisation of the B29, and I wouldn't mind betting that one hit from a 128 will destroy any heavy bomber in the vast majority of cases. Also I do not deny that these guns can reach the altitudes in question, I never have denied it. However, the higher you are the safer you are as accuracy decreases with height and its more difficult to set the fuses. This was one of the reasons why Lancasters were preferred over Halifax's and Lancaster II's for that matter.
b) There is of course nothing to stop the Germans waiting for the bombers at altitude after being warned by radar reports. That said though, it will take longer for the fighters to gain that altitude, giving the escorts more time to intercept them, also at altitude the defending fighters were less effective. 190's would struggle at that altitude and the 109's would have to go with their built in weapons as the extra weight of external guns would cause difficulty at altitude. Me262's would have a huge advantage but were very limited in their operational numbers, plus of course the few 152's. The US could and would have provided considerable numbers of escorts and their fighters were very effective at altitude.
This plus the fact that the B29 would spend less time in the danger zone would result in fewer casualties.
c) Better Defence
The B17G had three turrets plus the rear position and of course the hand operated weapons. The Hand operated guns were largely ineffective giving eight effective weapons. Note the rear position had limited arc of fire so it could be argued that this should be reduced but I am ignoring that.
The B24 had four turrets including the rear and a couple of hand held guns, again giving eight effective weapons.
The B29 had five turrets giving an effective defensive armament of twelve weapons. I am ignoring the 20mm in the rear as it was often taken out or it tended to jam.
So we start off with a 50% increase in the firepower. It was better defended.
Then we look at the other improvements. As to evidence that the modernised fire control system was better than the traditonal system I frankly have no idea, and don't pretend to. I do however think it unlikely that it was worse than the traditional turret so we still have a 50% increase.
My personal view is that it probably was more effective because all armed bombers went for some form of central guidence, even the radar guided turrets in some Lancasters was a centrally controled. In the B29 it also gave flexibility as one gunner could control more than one turret if the situation demanded it.
So to sum up at worst it was a 50% increase in firepower and at best it was a lot better, but it was still an increase.
Even if you disagree with all the above and say it was no better than the B17/B24. The B29 still carried a hell of a lot more bombs a longer distance and therefore, was more effective than either of the others.