I am not sure what you are asking, but I think that I was saying that the 500 is just too big for a nature walk, a bit uncomfortable to shoot handheld. The 300 with the 2X gives a 600mm f/5.6 that is still sharp and is easily handheld, the 500 really wants to be on a tripod with a gimbal. The 300 is without a doubt the sharper of the two lenses but tele extenders do impair the IQ, I have not yet done comparison shooting of the 300 with the 2X vs the 500 by itself or the 1.4X extender.... I have time but like I said, a 500mm lens is a big piece of glass and very imbalanced on the camera. The image below is the 500 with an extender. It is too much lens to take on a walk.

Before buying this lens I did a bit of research on it, it is said by many that it is Canon's sharpest lens. I know that Regis who has owned many of Canon's long white primes had moved to this lens, after talking to him I went ahead and made the purchase. As far as the claim that it is Canon's sharpest lens? I don't doubt that may be true but it is also a pretty short lens for birding and nature so extenders will almost have to be used in many situations. I have been told that even with a 2X extender this lens is sharper than the Canon 600mm, I am not sure that I believe that to be true but I do believe that with the 2X extender this lens is still quite awesome. I own a 500mm mark I and can share some of the advantages of the 300mm lens vs the 500mm lens. Even with the 2X extender attached the 300 is much lighter and more balanced than my 500mm on the camera, while it is true that the 500 can be shot handheld it is a lot of work and not something that most would want to do for any extended period of time, the 300 however is not much different than using the Canon 100-400, a bit heavier but nothing like the 500mm. With a f/2.8 max aperture and Canon's updated focusing system this lens is quick to focus and is spot on.

As far as IQ, below please find an image of a Cardinal the first image is the image that the second image is cropped out of. You can see that even at distance where the subject is presented very small in the image that the amount you can crop is only limited by the sensor, the lens will out perform many of the sensors of the cameras to which it is mounted, this image came from a 5DSR, I am certain that the lens will out resolve the sensor on my 5DIV.