I am no pro by no means.but have been loading for some time now.when loading a fire formed case I just neck size it.and i haven't seen much of a change in the load,other than not haveing to full length.the only change I get is when I change the weight or coal of the load.or even the bullet.
now with that said.the onley way I think one will see a huge change in the load is when one is fire firming to an improved case or even a wildcat.when I use to fire form for my 300 ackley.when doing this I was useing a 300 h &h case and it is a really different case,but was able to use it.with the case being so differant I used powder,primers,and bullets to just make a case in a 300 ackley.so I spent alot of money in case forming.but thats the price I was willing to pay.and said to say that I just learned a new way of fire forming.that cost alot less.
so neck size them and shot,with confidence.I don't think it will change anything.but I wouldn't be surprised that some or these pros here will tell you different.like I said I'm not a pro.this is just what I have experienced when loading a fire formed case.I hope that I am right or even close to what you are asking for an answer.
John