InLine Fans - Does CFM really matter if the exhaust length is short?

Quick Question for those Who have done this? did you have to remove the internal fan or just tell the GF that you had a external filter attached in the app? I have a inline 195cfm fan on the way and I want to make sure I have all the needed parts to do the job. I also sit very close to the window and mu Duct is a straight shot to the window with 14" of space.

Thanks