Actually 78MW is the sum of the power ratings of all the power-generating units on the Zumwalt, not what is being outputted from the induction motors. As for hull form, the 055's very standard hull form is certainly far more efficiently designed for speed than the Zumwalt, the Daring, or the QE. The Daring can go 30+ knots with two 20MW induction motors. Possibly the 055 can accomplish the same with two 20MW induction motors, given its significantly higher L:B ratio (8.67:1 vs 7.19:1). Or maybe two 25MW motors. Both should be within the limits of two QC-280s, leaving the smaller GT generators to supply power to the rest of the ship.The 78MW for the Zumwalt is what the drive motors driving the shafts can produce, the total output from the gensets is more, given its non-standard hull form it's probably all needed to get to flank. The QE "only" has 80MW driving it's props because the largest induction motors they had were 20MW ones and because of penny pinching it only has 2 shafts (can only shove so much power down a shaft) they used a bulbous bow to get it's top speed to a nominal 26kt adequate but would need an extra shaft to get past 30kt.
Hull form rather than displacement plays a big part in a ships speed a 40+K ton Nelson class battleship got tp 23kt with just 34MW! but I digress, Don't think they'll get away with just 2 QC-280s (56MW minus generation and transmission losses means robbing a 10MW from elsewhere) so probably need at least 3 (easily doable since they aren't hooked up to a shaft) but that really depends on what size electric motors they have, no point generating excess that can't be consumed, conversely no point not fitting enough. They could swap out a couple of the QC-280s for diesels which are more flexible (don't have to run them a full tilt to be efficient) but for maintenance probably easier to stick with all GTs.