With the exception of a few nutjob admirals, Starfleet and the UFP don't want a war with the Klingons. With anyone really. So, here's a ship designed by a rogue admiral, packed with BFG's and a warpdrive that can run circles around the Enterprise, one of the newest ships in the fleet. A ship made for war.
How the heck would the Klingons react when you actually put that ship into active production?
There's no reason the Klingons would ever need to see that ship - unless war has already broken out. Once that happens, THEN you deploy Vengeance
-class ships to the front lines. But until that time, keep them well within the Federation's borders, ideally within a secure facility, where the Klingons won't even know they're there. Keep them there until war already breaks out. Only then do you let them out.