I didn't see where Kelly Neuman mentioned his specific preferred rod weight for fishing his streamers. I did see that he talks about his custom streamer rods, but he gives a range of 6 to 8 weight rods.
I'm still relatively new to fly fishing, but I study something new about it every day. I'd agree with you that there are good casters that have taken the time, I wouldn't say trouble, to learn to double haul, but most casting instructors will tell you that the double haul is one of the last techniques a fly fisherman should learn. I have spent some time with a Master Instructor (he's a good friend of mine). He has no problem picking up his 9# Fenwick HMX with 400 grain line spooled up to throw Kelly Neuman's bigger, heavier flies. If he's throwing Gallup's streamers, he'll still use a heavy, stiff rod to control that line weight even though Gallup's streamers aren't typically as heavy as Neuman's. He preaches that the rod weight and speed must match the approach to fishing more so than the fish themselves. I can cast a heavy 8" fly with floating line in my yard (or stream) all day long on my 6# Z-Axis. But if I'm fishing fast water, and I need to strip that line through a deep hole to get it into the feed zone of a fish, I'll switch line weight and rod weight/speed to manage the fly and line. At this point early in my fly fishing career, my goal is to use whatever gear it takes (and that I can afford) to give myself the best chance to catch fish. As I continue to learn, I'll change my approach (and gear) to continue to give myself the best opportunity to catch fish.
I'm sorry I turned John's topic about his interest in Gallup's flies into a discussion of rod/line weight needed to cast them. Great ties John.