I checked out the wife's Tahoe last night which has the adjustable pedals and hers actually had more play than mine. I got it all shimmed up so I 'll see if she notices a difference or not, but she didn't squeal tire out of the garage this morning so she probably won't even notice lol.
Reardiff, I don't think you can over shim it within reason, just try to get close to the right thickness and pull up slightly on the pedal to slide in your material and let the assembly hold it in you shouldn't break anything unless you go all magilla gorilla on it lol.