Yes, you may get near 400 miles (maybe including reserve-- 10% is the bare minimum. Also you may not have 100 gallons usable in the tank--often there is another 5 to 10% loss there. ) The Boat Test.com (which most feel) is extremely optimistic, because the boats are extremely light, partial fuel, no gear, minimal water). With 150 hp Honda on the test boat (The Yamaha would be very close, within a few %.)
At 2500 RPM 8.8 mph 7.7 knots 1.9 gal/hour 4.6 Miles per gal 4.0 nautical m/g 446 statute miles range 388 nautical miles range.
At 1500 RPM 5.9 mph 5.1 knots 0.9 gal/hour 6.6 miles per gallon and 631 statute miles range. Once you get less than a gallon an hour any of the flow meters become inaccurate. The only way to get really accurate use rate is to use measured fuel, rather than a flow transducer.
As you go on a cruise--especially one such as the "great loop", the conditions are going to be anything but ideal. Most likely the boat will have more than minimal gear. Even boat wakes will decrease your mileage, there are currents, head winds, seas, etc.
I have done over half of the Loop--and at a very slow pace. One who really absorbs the trip, gains an in depth view into "Americana"--far from being a tourist. It is the people, the customs, the small diner, the 4th of July Parade in a small town in the Carolinas. Since the Loop is most likely a once in the lifetime event--make the most of it. It is more than a bucket list item. There are many safe places to leave the boat and then you can resume the trip.
I live on the Water in Pensacola, and hurricanes can come as early as June, although they are most likely in Sept. We don't hesitate to be gone, with the knowledge that from almost anywhere in the US we can get back to our home, or have some put up the hurricane shutters.
In any case, enjoy the trip--The C Dory 25 is a great boat for the trip!