I don't think there's a lot of value in starting the old "BBs are obsolete in WW2/1944" argument again - Denkt's focussed the thread on a 36 knot battleship, and much safer for it if it keeps talking about that.
As the naval buffs will tell you, it gets exponentially more expensive to go faster, the larger a ship is. So taking a 40,000+ ton behemoth like an Iowa to 36 knots should mean that a substantial portion of it's tonnage is used up in its machinery, leaving less room for armour and armament. I'd expect it to be possible to build something like that, but I can't imagine it being sensible until nuclear power revolutionised machinery for large warships.
That said, as well posted by Antless:
This is not a WWII naval simulator. Slightly inaccurate numbers are entirely to be expected (and will become more over time as balancing will set in). It's not about the hard numbers, but the way they compare to other numbers (like the speed of battleships to the speed of other naval vessels) and added to that a lot of wiggle room to make more paths viable.
It's no real biggy as long as all the relativities match up, and I'm sure people looking for more precision on the naval things can mod it in (part of the work on a refined naval tech tree I'm interested in is in more closely relating the in-game values with actual historical values).
What would bother me more, personally, is if the 30 knots/36 knots is its cruising rather than flank speed!