I've read of Defoor's observations in a class AAR from a very experienced pistolero. The context is Defoor sees a lot of students shooting Glocks. He shoots a lot of pistol rounds through Glocks. And his estimate is that the 17 on average improves a shooters performance at his class work by about that amount. In my experience it's valid. And while I love the 19 and we have five of them across the household, I think a powerful argument can be made for a 17 and 26. I am stunned by how close I can perform with a 26 vs a 19. Whereas the gap up to the 17 (advantage 17) is greater for me than the gap from 26 to 19.