Re: .325 wsm at 1000yds?
Roy pretty much hit on what I was thinking (and then some!). There aren't any 8mm bullets available that have a very high ballistic coefficient--I think the 220 gr Sierra is probably the highest at around .525 and it is too long to be efficiently used in that short case.
If you really want to stretch out your reach and are considering a rebarrel anyway, you might want to consider either the 7mm WSM or the 300 WSM and maybe a custom throat. There are excellent high BC bullets available for both of those calibers and I think you would find that they would have much less wind drift (most importantly) and drop than the 325 WSM and would arrive at the target packing a lot more energy at long range.
You could also consider keeping the 325, a nice caliber, and pick up an action cheap for a build from scratch in a better long range chambering on a long action.
Best of luck!
The critters have to win every time, I only have to win once.
NRA Life Member