Which players are you referring to? Our line up features very few players who were established when they joined us, IMO. Frimpong, Palacios, Tapsoba, Tah, Hincapie, Kossounou, Boniface, Wirtz, Adli, Tella and Hlozek were at best established in second or third tier leagues and most of them not even that.
Hm... I'll just go through the starting eleven against Freiburg and right now I am not sure if the result confirms me or proves me wrong...
Hradecky: Came as a DFB Cup winner. I don't see a significant difference between him at Frankfurt and Leverkusen
Tah: Arrived 8 (!) years ago. Made the step from being a promising youngster for Hamburg to an established top player in Leverkusen.
Hincapie: arrived as a young talent, improved a lot in a short time.
Stanisic: On loan from Bayern, was a pretty reliable backup for them and is the same for you/currently even a starter
Palacios: Was a very good player in Argentina, made the step up to the Bundesliga well
Xhaka: Was known as a top midfielder in the Bundesliga already a decade ago. Nothing about him changed when he arrived from Arsenal
Grimaldo: Really flourished, but he already was a top player in Portugal
Frimpong: Young player who developed a lot
Wirtz: just as Frimpong
Hlozek: not up to speed in the Bundesliga as he was before his transfer. Doing reasonably well but no impressive development as of now
Schick: Made a step up in performances after moving to Leverkusen
So it's definitely Wirtz, Frimpong and Hincapie I see as currently a lot improved players. I probably should also add Tah, I had completely forgotten that he went there so early in his career. A bit more then expected, but still less than Schmelzer, Hummels, Subotic, Großkreutz, Sahin, Bender, Pisczek, Götze, Lewandowski and Kagawa who I had in mind as a comparison.
The rest also developed well in Leverkusen (not surprising, otherwise they wouldn't be starting), but not to that level. And that "reasonable" development is more easily replicated with other players than the big development jumps and successes.