With the 600II and a Wimberley, I routinely get tack-sharp images at SSs of 1/640 and above. Being careful, of course. But with the 2X I much more often see what I assume is "ringing," presumably from mirror slap, even with the "silent shutter" mode of the 5D3. Stabilization needs to increase with something like the square of the focal length. I suspect a beanbag or hand holding (if you are Superman) avoids this issue, or some sort of dual support at the front end. (This assessment is, of course, from pixel peeping.) I can't speak for the Mongoose but if it's actually better I'd get one. But I would need evidence, not testimony. It's still a center-mount. And a side-mount scares me. Sort of like riding side-saddle. (Don't try it on rough ground.)
With a 500 plus converter, I don't think you can go wrong with either one.
Me, I'm still old school and will probably always use the Wimberley. I'm also with Diane
about the side-mount. I know its probably a 100% safe as long as you make sure
everything is connected correctly, but I just feel more at peace with a Wimberley.
If weight is an issue, I think the Mongoose is about a pound lighter.
My concern with the side mount is my being able to support the weight getting a heavy lens on and off. I'm not exactly a weightlifter. I feel more secure with something under the lens for support.
with my monopod When I mount my 1DX 600f4mkII +1.4tc to my side mount RH-1 I simply stand the lens on the hood and bring the side mount /monopod to the lens foot this is quite an easy way to do it and you can clearly see you are in the clamp with the foot, then I pick the lens up more than the monopod this ensures the clamp is set right.
Rob.