"In a Rem. 700, 30.06 production rifle with only an action bedding job, how much could i really gain by neck sizing only?"
If nothing else you save a lot of time neck sizing. No need to lube and then clean off the lube. It also works the brass less and should extend case life. I read (on the internet so it must be true) that a lot of the benchrest accuracy loonies full length resize so that internal case capacity is the same since the case capacity of a neck sized case can vary depending on how fast the case cooled after firing. That sounds a little excessive to me but maybe the question is how much would you really gain by full length resizing. My guess is not much.