I wanted to write an initial answer to kkck yesterday, but the site went down for a short time and i to bed. I will rather catch on with the current posts now. :P
Taking this piece of information into consideration, your conclusion of Shanks having twice strength or willpower or whatever is just simply wrong. Luffy on FI was just strong enough (however you want to define strength in this context) to make 50.000 people faint because those were the only ones so weak that you could not even consider them fodder. Oda pretty much introduced a new level of fighters and that´s a level even under fodder. Now, Shanks making all 100.000 faint does not equate to him being twice as strong as Luffy, it just means that the gap between his strength and all 100.000 was big enough for them to be effected by CoC. How much Luffy has to grow to reach that strength is not given at all.
Even if it's nitpicking, since it's assumption alltogether and we can only do with the english translation, you conclusion is not entirely right. It's nice to see the complete Quote though.
In exactly the same place and exact same situation as Luffy, Shanks or Rayleigh may have been capable of knocking out all 100,000.
This is a single statement regarding the question, saying the Haki of Shanks and Rayleigh may (he's not even putting a sole number on it) be able to. This means that there is indeed some kind of cap to the actual number of people you can knock out, which is limited by the sole strength of your Haki.
Oda then goes on about how the process of overpowering someone works in detail. This, in short, only says that despite the strenght of your Haki you also have to have a vast difference in power to succeed.
If you would raise the individual power of every fighter from the 100.000, the number of people Luffy would've knocked out would go down to 0 rather fast, while Shanks would still be able to keep his record entirely. This doesn't mean he is
that much stronger. It rather lays in the defined concept of how Haki works, that Shanks as one of the most powerful individual can pull off a sufficient power gap on much more individual people.
I guess we now have to all learn japanese and then discuss on. :P I'm not saying you're wrong, it's just one of those matters of interpretation where you can sport a whole different opinion, easily. All in all we'll never really know at all. :P
Do you wanna know why it was half? Because it was the damn middle of the journey, the middle of the Grandline, it's like a progress bar in a game. Maybe. :P
Anyway it was about haki, not strength, so not saying much about the power level.
This is not that a bold statement at all, given one of the longer shots in One Piece is Luffys "hunt" for Shanks, to fullfill the promise. So he's about half way through and achieved half of the desired power. Again, it fits in a rather simple but cunning way.
According to Rayleigh, while this type of Haki cannot be attained through training, it can be improved through training, as it grows as the user's spirit grows.
In the translation on Mangareader it also says: It is the user's very spirit manifest. It can only be strengthened by the individual's person growth.
This also means it's not a sole expression of willpower, but also a complete package out of cunning, experience emotions and so on. Given Luffy was more of a boy and now is a young man it's not that surprising that he's inferior to Shanks in that matter who is both, older and stronger. He also has much longer journey behind him.