The 264 would be a little faster but not by alot.As Im sure you know it headspaces off the belt not the shoulder.Probally not quite as accurate as the 284. Now if you necked down that 7mm stw then you would be talking 6.5 velocity.Barrel life,another question.
If you decide on the 6.5 Rem you can head space the round off the shoulder after firing the brass. When you reload color the neck with a black magic marker. Start with the die set high and just progressively wind it down until the sizing marks on the neck meet the crease of the neck shoulder junction. I usually do this with a full length sizing die due to neck sizing dies(with the exception of the Redding competition neck sizer) not referencing the case body to the neck which I find raises heck with concentricity problem from neck to case body walls.