Nitpicking a bit… 300W is 300W. A rider exerts the same energy to make 300W regardless of suspension. What matters is how fast that 300W makes the bike. The more losses you have due to drivetrain, suspension, tires (and whatever else), the slower the bike will be for a given output from the rider.
But, your main point is correct - if the rider made the same power, and one bike was faster, then that bike was more efficient transferring that power into forward motion.