The obvious thing to do now would be to try and make a Capitani Romani-style ship

What would a "Capitani Romani" style ultra-fast cruiser look like? It would be small. With a standard displacement of about 3750 tons, the normal displacement should be about 425o tons. The design would be moderately larger than my GB/CL/1921e ship. I think that dimensions should be 462ft x 45ft x 16ft with coefficients of Cp=0.66 and Cm=0.86. The power plant should be very lightweight but high power. I need to experiment to see what can be done. For my design, I would have 6-5.5in/50 guns (510 lbs broadside). It probably should have 8 guns, or should have 8-5.1in/50 guns. I'll have to see how this works in my program. The answer is that I was able to make the design work, but with a very high power-weight ratio. The revised dimensions are 462ft x 43ft x 13.6ft. The hull depth is 30.5ft and the normal displacement is 4,184 tons. The ship can reach 41 knots at normal displacement and only 36.8 knots at a deep load displacement of 5569 tons. The stability is adequate, as the metacentric height at normal load is 2.2ft and 3.96ft at deep load.