Throughout reading comments in the NV group, I see discussions of speed and aperture. What I am unsure of is what helps the most. From my personal experience it seems like aperture helps a lot. But then I wonder if that's just be getting a bit more image scale because of a longer focal length. For instance, I see a pretty obvious improvement going from my 6 inch F4 to my 10 inch F4.7. And I'm not saying the 6 inch F4 is bad because it's actually quite good. But the 10 inch does better on almost every object (as long as it fits in FOV).
The times I notice the speed benefit is when using something like my 4 inch F7.7 which is an excellent scope, but doesn't do too well on NV nebulas...I have not tried it too much otherwise. That being said, some of my best views with the NV has been with the 8 inch F10 on Globulars and smaller DSOs. It worked so good I was super glad I didn't sell that scope as I was trying to make up for the NV purchases. I have not tried the 8 inch F10 on many larger nebulas, but I want to give it a bit of a try with maybe my 12nm HA filter.
I think NV is a tricky dance of speed, aperture, and framing objects.
A bit of discussion on framing objects and why I think having many scopes really helps. Really large nebulas need a wide field of view. Even the 6 inch F4 needs some serious reduction to fit some of the larger nebulas in the view. However, seeing some detail on objects requires the 8 inch F10 for globulars and the 10 inch F4.7 for getting close enough views of the Eagle Nebula and the Horse Head Nebula. So really, my personal opinion is having multiple scopes is pretty important.