Tim,
I did buy a set of RCBS neck size dies. Big mistake. I've never seen dies give me worse run out.
What was the runout "before" you neck sized?
The neck sizer will not hold the body tight like the FL die will, so consequently it will not correct any runout that already exists. Your FL die should, with out an expander produce near "perfect" cases and correct any runout problems in your brass from a bad chamber.
My 300 Ultra chambers neck area is eccentric in the order of + .005" at the 2 o'clock position so all I can do is FL size or index the cartridges to gain some consistancy. I mark the casehead at 3 O'clock and index the neck sized cases right now. I'm getting less than .005" runout way up on the bullets nose like this. I've not done a chamber cast to see if the neck or the body is lined up with the bore yet but one of them isn't.
With the Redding dies, which I still prefer, I've noticed the decapping stem will orbit the center and "never" be centered. If you leave the lock nut loose on top it will pull it closer to center as you draw the case neck back over the expander, but it isn't perfect. Redding dropped the ball with keeping their seating stems concentric. They are so far off it simply makes it impossible. Example, I use an RCBS Casemaster to check runout. if you take the seating stem out of the die, remove the decapping pin and collar, lay it in the supports, one one each end of the rod itself with the pin end to the left up against the stop and indicate off the flat just beneath the knurled head. There is always about .010" - .015" runout on the threaded adjustment head and it's only 1.0" in length at that point too. You can double that runout figure because the rod length is about that on the longer cases down to the expander ball. The rod is pressed in a bore that is not paralell for some reason or another, the threaded part is fine it's just the bore that holds the stem that is way off. If the threads on the adjuster are loose enough (lockring left loose), the expander will pull over close to center when with drawing the case but that's as good as it gets. The only way you can tune a stem to center up is to hope that the threads are unparalell also, causing the orbiting expander to intersect the bore centerline at some point. This is highly unlikely considering the threads always seem on center perfectly.
The carbide expanders are ball shaped almost and float on the stem with the hole in them larger than the stem itself, that coupled with leaving the lockring loose is the best way I've found to use the expander "if you must". I usually only use them to iron out dented necks if needed, I try to avoid using them. The smaller surface area on the carbide as well as it being harder usually doen't require lube, but if necks get harder or need to be opened up too much cause you die squeezes them down too far you'll still want lube, pressure to draw over the ball will tell you. I have had to use the ball on my Ultra with my neck bushing die because my bushing is too small now by about .003" which would leave me .006" tension and it takes some effort without lube but has not pulled necks up enough to cause tight headspace yet (3 times) using no lube. I get less than .001" runout using my FL dies.