Good idea to shorten it. I wouldn't go as much as cutting it in half.I will shorten my 24" extension to 12" and go from there.
Take a few inches at a time. You can always take it off, it's hard to put back on. When you're done you can do a biscuit test for a few hours and see how she feels. I have the feeling you're going to find you'll be able to dial in the airflow but you'll amplify the cooking from the bottom up.