The magic of the GL1800 engine is that it is big and makes a lot of torque without having to spin fast, so it also lasts a long time. For useful power, you want a torque curve that jumps up off idle, goes high and stays flat as long as possible...like this one. Very few motorcycle engines will make 90 ft lbs...the GL1800 makes it from 1500-5500 RPM's. Notice also that the horsepower builds in a perfect straight line to 5500, making throttle response very predictable.
There's not much point in reving it past 5,500, since everyting is dropping after that.
They could have made it spin faster and breath better and it could make a lot more horsepower, but it would have to give up low end torque and durability. But, at what RPM do you spend most of your time on a touring bike?
(There is no replacement for displacement.)