I would stay in the Sevas till BB3 IM or Kron because of the following:

Better OH with at level engies. Longer and Faster.
Better Range.
Better Damage. (they arent the same guns)
Better Speed. (I am at 43knots)
Better crew slots. 11 vs 12 total
Submergence(Ships tuffness) seems better with the Sevas too but thats hard to tell b/c of the size of
the Gangut.

The only things that are worse:

The spread. I can live with hitting 2 ships at the same time. ;-) Damage is damage.
Sevas is bigger.

You can use the trip 14"s on the Izmail...

and how many sailors can the izmail have on board?
sevas can carry 12 sailors...

and izmail is bigger than sevast... that s a disadvantage

but izmail has 4x trips 12"
same as sevas....
what is better on izmail?
range? speed? displacement?

Gangut<PBB<Izmail<BB3s IMO.

Someone people prefer the PBB to all BB2/3s.