I don't think 30 RPM is very feasible in 1940. For example, the semi-automatic(essentially all automatic guns in that caliber class was semi:s) Bofors 120mm M/1942 was capable of 20 RPM. The famous 5"/38 Mk.12 could reach 18 RPM, with appropriate gun mounting.
Also, I can't avoid thinking those gun barrels look... huge. Guesstimate from just looking is that they are around 70 calibers long, which is... very much. A 105mm gun could have that length, but in 1940-ish 5" guns were rarely longer than 55 cal, often shorter.
Except for that, however, I very much like the appearance of this destroyer. The story is intersting as well.