Just noticed this thread on clamshell doors. Back in the 90's, I was flying a 690 with doors that were later removed. I'm kind of nerdy when it comes to tracking any changes which affect cruise/top speed. To my surprise, the loss of doors had no detectable affect on cruise speed.
I can't speak to any changes to noise/vibration as I only sat in the front with my headset on.
Funny how a little thing like gap seals makes a difference, and a big thing like gear doors does not.