Iowa a poor gun platform? As obsolete ships go... The Iowa class of BB was far superior to the Bismarck class. Iowa class had superior radar, speed, armor, weapons to the Bismarck class.
As a great seakeeper... not sure what you mean, but if you mean endurance- range is more than you think. If you mean sealane dominance- Carriers can control huge swaths of ocean, whereas the Iowa was limited to 32 miles or so.
Well as the US had been building ships throughout the 20's and 30's, and the Iowas were at the end of a line of development, whereas Bismarck was based on the old WWI Bayern hull (hence the poor underwater protection and weakness in terms of shock damage) I would be most surprised if it were otherwise.
As for seakeeping, the Iowas did experience some, ah, submarine properties and were not particularly stable as a platform for her guns. The 'wetter' a ship was and the greater her tendency to roll the harder it was to use the main armament to engage targets.
My money for a balanced battleship is Vanguard. By the USN's own postwar admission the Mk1 15" was the best all round gun, armour protection was good, seakeeping reasonable and as a gun platform pretty stable under most conditions. Thing is in HoI, as Vanguard used the turrets taken off Courageous and Glorious, you could realistically only build one of them!
We'll never know of course. Neither class were exposed to the classic capital ship encounter. Bismarck was extremely lucky in the Denmark Strait encounter in a similar way to Scharnhorst being very unlucky at North Cape, but look at Bismarck's gunnery in her encounter with KGV and Rodney it is pretty poor indeed.
Another issue is how to deal with the German limits on engaging ships armed with 15" guns that limited S and G on their early 1941 foray into the Atlantic. In two cases they broke off from attacking convoys due to the presence of an elderly British battleship: in the first instance Ramilles, the second Malaya. Particularly in the first case, when the captain of Gniesenau offered to draw off Ramilles so Scharnhorst could get at the convoys (Lutjens vetoed the idea) in HoI an elderly R class (whose unmodernised turrets limited the range of her 15" to less than the German's 11") would in no way limit any attack. In short Ramilles would have been toast. But the political orders issued to avoid the loss of prestige similar to that suffered when the Graf Spee went down took precedence.
Now, no HoI player would take this option. Indeed in HoI it is impossible. But it happened, and had the effect of hampering the Kriegsmarine in its operations. Can it be modelled? Should fascist countries suffer for such a loss of prestige that goes with the sinking of a warship? After all, 'Deutschland' became 'Lutzow' for a reason. The Democracies certainly did not suffer as much; as both the USN and RN had pretty significant losses with far less political impact.
K