It's actually hard to answer. Some fiberglass dishes do OK and some don't. Most of them were made in the 80's and there were many manufacturers, some did good and some didn't, so somery, you are correct in what you say, but it doesn't apply to all the fiberglass dishes. I have one I purchased in 79 for C-band, ku was not a factor. It holds it's own pretty good for ku, even though it was designed for C-band. I have seen a higher quality signal on a 30" ku dish (compared to my 10' fiberglass), but I've also seen that 30" ku dish lose the signal, while the 10' fiberglass still has a solid signal. So, like I said, it's hard to answer?
Al