Bill Gates does a lot of his observing from high elevation sites in Arizona which often have superb conditions, so his sighting of Cas A with an 9.25 inch SCT was probably made in ideal conditions. On the other hand, I've seen it with a 16 inch f/4 Newtonian under 21 .2-ish sky fairly easily using an OIII filter. My guess is that sky transparency makes a huge difference for this object.

I'm eager to give Cas A a shot with my 8 inch Dob some fine night, but I'd really like to see it through Jimi's 48!