It depends on the numbers, of course. If the 95% of kids only learned 1% more, it wouldn't be worth it.
It's notoriously hard to measure those numbers because everything is confounded by everything else and experiments take decades to run. But I think 20% is plausible and 1% isn't.
Again, it all depends on where the 5% end up. You're not allowed to take them out back and shoot them like an aggressive dog, so are you going to look after them and give them some kind of a future, or are you going to throw kids out onto the street to grow up into career criminals?
Depending on assumptions, it is easy to argue that money is best spent accelerating our brightest minds and giving extra support for the most at-risk, letting the middle atrophy. Or it is best to treat everyone equally, which certainly leaves the top underfulfilled and the bottom underprepared. It's just spitballing.
Your numbers are all invented whole cloth; please don't try to math with them.
More precisely: Why should I believe that the remaining kids learn more? In my experience, everyone learns more when they teach, as it forces them to shore up their assumptions. (When you learn something yourself, you learn one wrong way to do it. When you teach, you learn thirty wrong ways.) Having kids who think differently around for peer exchange is then a good thing overall. The struggling kids have stronger peers around to teach them, and the stronger kids have more chances to teach.