I use 338 Win Mag brass to form 7mm Rem Mag brass. The necks do thicken up some, and this reduces some of the slop between the case neck OD and the chamber throat ID on my two factory rifles. I outside neck turn with a K&M tool. I do basically exactly what I understand you're thinking about doing. Don't ask me to prove that it improves accuracy and if it's worth the time and effort. I don't shoot enough to prove it to myself, let along others, and I'm not interested in wasting barrel life trying to prove that point. Competition shooters like tight-necked chambers, so what could it hurt to move in that direction with my factory chambers. There's still plenty of case neck to throat separation with factory chambers even with necked down & thicker case necks. Just less than if I was running 7mm RM brass. I also anneal the case necks after sizing down. Do these 338 WM cased reloads shoot well out of my two Tikka T3 factory rifles/barrels? And do I have low case neck runout? Yes & yes. I do have to use larger ID Redding neck bushings with the parent 338 > 7mm cases than was required with 7mm Rem Mag brass. I've done this with both Federal 338 WM and Winchester 338 WM brands of brass. Before selecting the brass to use, I measured the outer diameter of loaded factory rounds to figure out which brand of brass typically had thicker case necks. Federal was thicker. I use Federal brass in one of my rifles, and the WW brass in the other so I know which shells go with which rifle.
Oh yeah, I periodically get some weird looks when I tell people I'm shooting 7mm RMs should they happen to look at my case heads and see them stamped 338 Win Mag.