@denobravo
I think what you are truly looking for, only you can provide. I personally don't think there is a single
meaningful answer to your question any of us can give you that will leave you 100% satisfied.
If you truly want to know take 25 pcs of brass run them through a stainless steel tumbler, anneal the necks, and create your load. Shoot it over the chronograph (hopefully one not effected by light) then re-anneal and shoot again with dirty necks. Compare the results. Boom, question answered.
But, I feel you aren't seeing the forest through the trees. As other shooters have said, ES is only valuable because it leads to more predictability, which leads to greater accuracy potential--which is the end goal for most if not all of us here. And personally, I focus on low standard deviations, ES is pretty meaningless to
me, but that's a whole 'nother discussion for another time.
Perfectly clean necks (ss tumbled) can have a major issue related to the bullet upon seating. The neck is so clean the bullet rubs against inside of the brass like a cylinder in a car without oil. What your left with is a bullet whose jacket has been deformed from its original form, it's torn or slightly shredded.
Not a big deal at most normal ranges, but at extreme ranges where ES/SD actually matters, it
could have an effect. It would be like pointing the tips...but in reverse...and opposite
And yes, you could use dry lube. But then why go to all the trouble to get clean necks just to make them dirty again. My advice to you would be either test for yourself so you know for sure or just get a few bronze brushes one caliber larger than the caliber you are shooting and run it down the neck and back up one time in between firing. This will knock off the large major gunk but leave enough for the bullet to seat smoothly. It's quick, easy, and effective.
My other piece of advice would be to shoot more often. The more you shoot the more you can
see the forest.