Its seems to that the 300 Pro is just a sticker for those using the 4,88 inch diameter case (the old version) aimed for lighter hulls. And the standard 300 will be delivered with the 5,44 case.
And I just read this again (note regarding the new Poseidon gearcase). I think the 300 pro is 300 hp only ..
7. Which 250 or 300 hp Verado should I buy?
If you’re running a single-engine boat that goes over 55 mph, such as a bass boat, a freshwater multi-species boat, flats/bay boat, or a pontoon boat, we recommend you go with the new 250 or 300 Verado Pro with the standard 4.8-inch gearcase.
If you’re running a larger, heavier single-engine boat with a top speed of less than 55 mph, or any multi-engine (dual, triple, or quad) boat, you’ll want to go with the 5.44-inch, 1.85 model. This larger gearcase will provide the added robustness and lift that these increasingly larger boats require. In addition, it will provide the shock-absorbing protection of the Flo Torq II hub system.
If you’re the type of offshore operator who continuously finds yourself in very rough seas, or if you’re a more aggressive boat driver who finds your propellers are frequently ventilating (the propellers are coming in and out of the water rapidly, creating significant load / unload on the engine’s drive train), you will want the 300 hp Mercury Verado with the silver 5.44-inch, 1.75 HD gearcase. The 5.44-inch, 1.75 HD gearcase provides a larger propeller shaft and the solid hub system that is more suited for this type of environment. It is important to note that this gearcase option requires the use of Mercury Racing’s 1.25-inch solid propeller hub kit (the HD gearcase is not compatible with the Flo Torq II shock-absorbing hub system).
9. Is there a performance difference between the 300 Pro FourStroke and the 300 Verado with the larger gearcase?
The actual power output of the 300 hp Verado is the same regardless of which gearcase is used. However, it is important to note that the 5.44-inch gearcase is significantly larger than the 4.8-inch gearcase. On some boats this will be advantageous because the added weight and lift of this gearcase will help increase boat speed. On smaller, lighter boats, the 4.8-inch gearcase will likely deliver higher top speeds due to lower hydrodynamic drag characteristics. Mercury recommends the Pro FourStroke variants for all single-engine boats that travel faster than 55 mph, including bass, multi-species, flats/bay, and tri-toons, for maximum performance.