Efficacy: the mRNA vaccines have a large margin for error with respect to dose. 55% of several times more than enough is still more than enough.
Danger: suppose the danger of broken-up mRNA to increase linearly with the amount given. Then the increase of danger would be by a factor of (100-55)/(100-78) ~= 2. The substantiated near-term danger of Pfizer is very low. We're at 5.2 million people vaxxed in Israel now… Twice of very low is very low. In order to argue for "too much" danger you have to argue either for non-linearity or for long-term high danger. If you argue for the latter then you already think the danger to be high, so there's no real difference between your view now and your view before. If you want to argue non-linearity of danger then you are arguing for a very remarkable biological coincidence. You also have to explain why the occasional 5x overdoses, which happen when a someone is given a whole vial by mistake, don't seem to be doing any harm. Never mind that it would have come out in the animal trials.