What integration of aircraft propulsion would be more efficient at sea level high speed flight? A turbofan, unducted fan or a turbojet?
Penguin ancestors took to the sea for food, not to set speed records.  There is absolutely no reason to design an aircraft to cruise at sea level.  Plus you take away a very important lifesaver in the event of engine failure, the glide.  Far better to fly in thinner air, where true airspeed can be pushed all the way to your Mach limit while the indicated airspeed is far lower.

Simply apply your graphs to higher altitudes for greatest efficiency, and, unless you plan to go over 400 knots, the turboprop should be your choice, unless you find a good mechanic for your turbo-compound radial.

The weather is also a bit smoother up there too.