[Suicide Junkie]

And my example shows something like - a battleship with 14" main guns set sail immediately after it construct, full speed toward battlefield. When it arrives, 18" main guns technology is already there.

Well, with all of the scientists in your empire working on the problem, and a two or three month travel time, that is not entirely unreasonable.

Rushing down tech areas to get the final uberweapon of that Category is one of the things I don't like and is on my list of things to mod out.

Higher research costs is only part of the equation. Having much longer tech trees, with diminishing returns is another. Adding Grid techs instead of linear techs will also help.
(EG: Multi tech areas: DUC damage, DUC accuracy, DUC durability, DUC mass production)

Then it would be more like "during the month you were at sea travelling to the front lines, the guns got obsoleted: we came up with a more durable design for the turret gears. It'll reduce the maintenance costs by 2%".