No, I doubt that in a DE boat that the OB would be nearly as economical as a modern diesel. Probably 35%-40% more fuel used.
Although an OB would be fast, when the weather intensifies and you are 40 miles from shore, I would far prefer to be in an IB diesel powered boat. That is if the there was room to service the engine.
If the weather intensifies 40 miles offshore, I don’t really want to be in a 26’ no matter what it’s powered with. I want to be in a 35’+. Whatever the propulsion, I would be very careful going a long way offshore in a 26’.