Airflow and shim question

Hey all

I just got 2 questions for the pros on airflow.

1 - I am planning on installing flexible ducting to get cool air to one of my 120mm fans. If I use 100mm ducting, would that hinder the airflow or is a 100mm clear duct good enough for the airflow that the fans are producing. The fans are standard at the moment.

2 - I read about the middle dead spot with fans being installed directly on a heatsink/radiator. How much difference would there be when adding a shim to extend the fan further out from the heatsink/radiator. I can't find any shims so would have to make one and don't want to really go to the effort if there is only a very minor difference.

Thanks
Alan