Do you have to turn necks or worry about shoulder brass flowing into the neck when you resize from .243 or .308 to 7mm-08?
I've converted primarily two kinds of 308. FC and some military kind with headstamp I can't remember.
The FC comes out with an OD neck diameter about 5-6 mics larger than my Winchester 7mm-08 brass. They have been working fine in my particular rifle, with both jacketed and even oversized cast bullets. There's no resistance on my bolt or pressures signs, yet, anyway.
The other military headstamp comes out only 2-3 mics larger. And the one BHA MATCH 308 I reformed measured only 1-2 mics larger.
So do I have to neck turn? Not yet. I think it was Clark that recently declared you can neck down to 7mm without neck turning as long as your chamber is in spec. You can neck down to .260 90% of the time. And to neck down a 308 to 243 would always require turning.
I have not checked for donuts in my 243 cases. I have only tried a handful with jacketed bullets, and only after the first sizing, so I don't know if they'll appear later. I use my 243 cases for cast bullets, which I seat above the bottom of the neck, anyway. I assume a donut wouldn't affect pressure unless you seat past the shoulder. And it would logically only form while you chamber the case (a FLR expander ball would push the extra brass to the OD of the neck until you cram it into the chamber; even if you neck size, the bullet would bulge the brass back out until you crammed it into the chamber), so I would think you should be able to feel it as significant stiffness on your bolt by the time it was dangerous. So
I don't really worry about donuts, but that probably doesn't answer your question.
FWIW, I find reforming 308 cases is a little easier than 243. (I do the 243 on the press, too; no fireforming). All I'm using is Frankford Arsenal spray lube, like normal, but I always apply some inside neck lube for both up and down sizing. (It's getting the expander ball back out of the thicker 308 cases that's the only real aggravation.)