The big difference between a KTM/Husky 450 and 500/501 is the larger displacement is done by stroking the motor rather than by a bigger bore. All four bikes have the same bore. But they get the 500/501 up to 510 cc with a longer stroke crankshaft. It changes the whole character of the motor from a harder hitting, faster revving engine into a much more torque oriented motor. Don't get me wrong, the 500/501 is still a friggin rocket. But it is a friendlier rocket when ridden at low and midrange rpm. Which is what it sounds like Honda did with with a heavier flywheel and other changes from the race bikes. Remember, KTM and Husky don't sell separate competition enduros. Their street legal 250/350/450/500 EXC/FE are their enduro race bikes. So, their 500/501 is actually the one "detuned" for street with a calmer power delivery. Though detuned is the wrong word because it is still crazy fast and a torque monster. It just revs a bit slower so it doesn't hit as hard as the 450's.