I cruise my R-27/OB at 30 to 38 mph when there's calm waters and no more than a 1 foot chop. At these speeds the R-27/OB is very stable and feels solid but does require some wheel effort turning when at speed. The F300 motor has tons of thrust (you can feel this with your bum meter when pushing the throttle lever forward and passengers will be very aware of this as well) and likes to run at and above 4500 RPM without any hesitation. I can achieve around 2 miles per gallon running at some 32 mph and engine at 4800 RPM in calm waters. The F300 can push the R-27/OB up to around 42 mph with engine at some 5800 RPM using some 1.7 miles per gallon.... again in calm waters. It is a fun boat to cruise in at high speeds but much care needs to be taken keeping a sharp eye lookout as at these high speeds things can happen very quickly.
Now having said all this above, one must mention how this R-27/OB will handle in rougher water conditions, such as 3+ foot waves, swells, following seas and wind. If these conditions are prevalent in your typical boating the R-27/OB will have to be slowed down.
Slowing down to say 10 mph in gnarly conditions (as you've mentioned) the F300 will be turning at around 2500 RPM and be burning 3 gallons per hour and doing some 2 miles per gallon. At this speed the R-27/OB will be bouncy, possibly uncomfortable and noise from water slapping the planing hull will be noticeable and this could be irritating for you. Compare this with a Ranger Tug R-29S with its semi-displacement hull and 300 HP Volvo inboard diesel (yes, it's some $60K to $80K more expensive) running in same water conditions just mentioned for the R-27/OB running at 10 mph and the difference in speed and comfort will be very different. The R-29S will easily outrun the R-27/OB's 10 mph while providing a much better and more comfortable ride.
Thus, if your water conditions are constantly 'gnarly' (as you've mentioned you will be having to deal with) a semi-displacement hull such as the R-29S has will give you a faster cruise speed with improved comfort and comparable fuel burn compared to the R-27/OB.
When I select to cruise in my PNW area with my R-27/OB I tend to select times when there little more than 1 to 2 foot waves and light winds so I can cruise at 30 to 38 mph with a comfortable ride with its planing hull. With a semi-displacement hull I would not be concerned at all with 3+ wave heights at all. In other words, the attention to weather and water conditions are far more important to be considered for using the R-27/OB with its planing hull compared to say the R-29S with its semi-displacement hull. In calm waters the R-27/OB will easily outpace the R-29S, but with bad/worse 'gnarly' water conditions the R-29S will outpace the R-27/OB and provide a much better and more comfortable ride.
Don't misunderstand me here.... the R-27/OB will handle most water conditions but care has to be taken when things get 'gnarly' and the boat needs to be slowed down.
Engine maintenance advice for you....
Make sure you have a nearby Yamaha F300 shop or whatever outboard model your R-27/OB comes with. The boat will need to be hauled out for engine servicing at the first 20 hrs of use and periodically thereafter.
I was recently (past two weeks) in Alaska - Kodiak Island and most of the smaller fishing boats were all Alu construction with twin 350 HP outboards on them... Yamaha and Suzuki brands. I went out with two of my grandchildren and my 7 yr old Wire Hair Fox Terrier on a 12' beam Trophy II all-Alu boat with 2x 350 HP Suzuki outboards for 5 hrs in maybe 1 to 2 foot chop and light winds to get close look at wildlife along shorelines and out further to get a close look at hump back whales and Orcas. The Captain pushed his boat along at a good clip of around 32 mph at times and the boat was awesomely stable with a huge roaster spewing out from the stern. This Captain typically takes fishermen out for day tours and charges some $300 for the first hr and then reduces it to $200 per hr thereafter as the twin Suzuki engines drink the fuel in a hurry when going between fishing ground areas. These Kodiak boats beat the heck out of their outboards and they live up to the task. I suspect the widespread use of outboards on the fishing boats is that engine maintenance is easier and less costly than for inboard engines.
Having an outboard is awesomely useful for controlling stern movements especially when docking. The R-27/OB has a bow thruster but there's no stern thruster so the outboard has to service controlling where the stern moves to. I have placed a red line on my Yamaha F300's cowling so it's immediately obvious to me for knowing the direction the engine is pointing.
