Bulbous bows make a difference at the margins. They don't compensate for fundamental differences in hull design. In the case of the South Dakota class, a major limiting factor was the form aft. This was fine for their designed speed, and was part of the tradeoff needed to get the desired armour and armament into a compact package. But going much faster just wasn't going to happen without massive reconstruction.
Adapting the Yamato class form wouldn't just entail 'adding a bulbous bow' - it would require adding about 50 feet to the ship's length, increasing beam somewhat, and would only gain about one knot. In exchange, hull weight would increase by about 10%, requiring a reduction of 2,000 tons elsewhere. For reference, the armament weighed 2,750 tons, its ammunition another 2,000 tons, and the protective scheme about 10,000 tons. So - how much do you want that extra knot?
None of this, incidentally, is even remotely feasible as an upgrade or conversion, except in the sense that the PURITAN was 'upgraded' by building a new ship and sliding it under the old one's nameplate.
