With the amount of guns those ships have and considering the shells they fire and the amount of shells they send downrange, any ship would be destroyed in seconds, in RL, a single cruise missile can sink an entire carrier if it cannot shot it down(perhaps a bad example in this case).
And considering that a ship of this size would probably weight hundreds upon thousands of tons, i consider its speed to be quite reasonable. Besides, they don't need to be faster, i see them more as a massive support ship/flagship to hide behind if things get tricky.
(advised not to hide behind Mk.IV as its shock wave hits friendly ships as well.)
I give my capitals a set number of HP, i start at the vanilla equivalent (say 10k), then add 5k for every ship that is bigger and then adjust it +/- to the point it neither feels overpowered nor underpowered.
I've boosted their HP a little except MK.I (doesn't need a buff) and IV because i don't want to give it more than it already has, since i boosted it in the last patch, they will come in like this.
-Vatican Mk.I = 12.000 HP " N/A "
-Vatican Mk.II = 17.500 HP "+ 2.000"
-Vatican Mk.III = 25.000 HP "+ 2.500"
-Vatican Mk.IV = 30.000 HP " N/A "
Better?
Now, if you feel that it was unnecessary or to much of a boost, please speak up.
https://www.dropbox.com/s/25dqknviop755wf/Valkyrians.zip <---- with new stats, OP also changed.