I have done similar testing in a few different rifles. One item you may be over looking is Bullet Runout, not neck runout, Bullet Runout. Different sizing dies can cause varying degrees of Runout. This may affect accuracy more than the sizing itself.
When I did my tests I Neck sized all the cases the same with the Lee Collet die. Then I FL and PFL sized with the Redding Body die. Only ammo with .001 or less Bullet runout was used for testing.