They are just numbers. And I think the size of the numbers are simply a means to illustrate the point better. I don't think the point was to argue the merit if the 80-87% are the correct numbers, but rather the general point. You're effectively arguing semantics. You're probably right that the range is not very accurate (as I said, it's clear they weren't supposed to be anything scientific), but it doesn't really change the overal point and argument - that being, that a car might pose the absolute limit on what is possible, but a driver might make enough of a difference to outclass another driver of a different (quicker) car. I think we had a similar topic covering these angles somewhere else not too long ago...
Anyway - I don't have a problem in using arbitrary numbers to highlight a specific point. Sometimes, it's the only way to explain a reasonable argument, and personally, I don't think what Manoah2u is suggesting to be that unreasonable. I might not agree with the exact numbers or difference (as I believe neither do you), but the overal point, I think is reasonable.
I certainly don't believe Senna could get 100% out of his car though. That to me, is a certain impossibility - because he raced during a time when cars were even more difficult to drive. More difficult = harder to reach the cars full potential - but at the same time, it would give any talented driver the opportunity to outshine other drivers by a higher margin. Exactly as Senna did, which is why he is still seen by many as the greatest of all time. And I think by the same logic, the same people rate Alonso as one of the best of this generation; Because he found himself driving in more difficult cars and is (therefore) outshining his team-mates by a higher factor which makes it seem a higher accomplishment relative to others. To some degree, I agree with this; It shows the class of Alonso and his talent - but by the same logic, I do think that the praise he sometimes gets is a bit exagerated, because he makes the car look worse than it perhaps is or is being messured with team-mates who are punching way below their weight.
It's hard to gauge how good he is, because quite frankly, you would have to pit him with a team-mate that isn't struggling. We can say with some certainty that he drove better than Massa and than Raikoennen currently, but it's difficult to assess how much he is getting out of his cars relative to the theoretical maximum. Then again; the fact that he keeps performing in pretty much any car you give him, simply shows the scope of his talent, which is undisputable.