I'm sorry but, yes you did:
and the rest
Hmm, where to start,
Regardless of the change in material, a resonant length x diameter combination will remain the same. Only the velocity factor would change the length and the velocity factor of air is so close to 1 at .9994 that it may as well be 1.
You say you would never be able to tell the difference on the other end, but if the driven element begins with 1-2dbd of gain and if that is increased by 5/8 parasitic elements, then how would you describe 2-3dbd of gain as insignificant? Never be able to tell? 
It might be all that is needed to barely make the contact through the noise or get over a jammer. Gain is gain and who's to say what amount it will provide until it's built and tested.
Why would you or anyone say that gain is insignificant?
Gain is gain, and I'll take all I can get.
Next you say it wouldn't be worth the size. HUH? 
A 22.5' wide beam with maybe 2-3dbd gain isn't worth the extra 27" on each side? Why not? That's no mechanical nightmare, I've seen 20m yagi beams that were a lot wider than that.
Gain is gain, and who's to say that an additional 2.25'/side isn't worth the gain?
I'll take all the gain I can get.
[I misinterpreted your statement about parasitic elements needing a matching network]
What does mutual coupling have to do with the price of tea in China? Any antenna with parasitic or multiple driven elements can be in danger of mutual coupling, that's why trial and error testing and software modeling exists.
As far as having to provide you a link to a 5/8 yagi by using the ARRL handbook, this 5/8 beam idea was initiated by Booty when he began this post so why don't you tell him to do so before he ever posts another idea?
Because that would be ridiculous, just like demanding I show something in the handbook just because I challenged your offhand claim it wouldn't work. How would you know if it would work or wouldn't, have you tried it? Have you built one and tested it so you are speaking from experience or are you just saying no because it's easy to be negative?
This post is about a different idea and the possibility it might work, the onus for proof is on you who emphatically stated "It wouldn't work if the driven is a 5/8" as per above quote #1.
WTF? This isn't a pissing contest, this is about an idea of Booty's that no one knows whether or not it will work until it's tried, so what is all this negativity and nay-saying as if you already know it can't work?
If some factual info exists which demonstrates it isn't viable then show us where it is, otherwise why be so cock-sure it isn't feasible?
