Does anyone currently own or have experience flying an ICON A5 aircraft?
They claim a range of 300 NM (555.6 km). I would like to know whether this is a theoretical calculated range, or a true/actual number?
I can't speak directly to the ICON A5 as I neither own nor fly one, but for every aircraft I'm familiar with the manufacturer's "book numbers" are generous theoretical values - for example, they typically assume flying perfectly straight-and-level in a no-wind condition, and getting the best possible fuel economy performance from the powerplant.
The Icon folks are openly honest about this on their specs page:
Performance specifications are estimates only., and range is certainly "Performance" in my book.
Personally I have no doubt that the A5 could manage 300NM under the conditions for which its designers did the math to arrive at that number - it's a slick little plane with a fuel-sipping engine - but they may have done a "to empty tanks" calculation (leaving you to account for the VFR fuel reserves you're legally required to have), and in the real world the 15-knot headwind you run into will substantially reduce your range (or, conversely, you can fly in the other direction and increase it).
As with all aircraft your mileage will, quite literally, vary depending on the day and direction of flight.