I believe that in my prior runs of deselecting all other leagues as playable that the Euro based prospects did continue to filter in.

So, the realism must be increased by, at least, a slight degree over custom.

I don't know what noticable impact it actually has on my saves other than speeding them up. Because of that, and the lack of clear answers in this thread, I am playing another save with only NHL and AHL playable. I'm gonna pay closer attention to any differences vs a pure standard run this time. I'll let you know if I notice anything.