I can positively state a riser does restrict some air flow. Does it matter, depends on what you want to do with your UDS. Just yesterday I did another test on mine. 10-12 degrees out, windy, single digit temps, snowing. Both 1 1/2" risers removed ( just twist them out) I was at 600 degrees, put em both back in, temp stopped climbing and settled back to 525/550 deg. Removed em and temp went back to 600 degs. Then I removed the lid, put a pizza, temp dropped to 450. I cooked 2 pizza's 12 minutes each about. During the first I reinstalled the risers for a couple minutes, just because and the temp settled at 500, removed the risers and temp was at 550 when removing the first pizza. My 2 risers, one is 16 inches, the other 12 inches. Also you have to have exhaust control if your shooting for higher temps. I have been playing with that also from the flat lid to the dome lid. I found it easier to work on the drums if you have a 2 wheeled wheel barrel, lay the drum in the wheel barrel it holds the drum at a decent height to work on for drilling , welding etc.