The .257 Wea mag is a very good round. I hav ebeen shooting one for about 25 years now, in a Mk V. I have taken everything from skunks and groundhogs to deer. Never had to put more than one round into any of them. You would not gain that much from the .25 AI over your 25-06. Another option is the .25 Gibbs, which is a ballistic twin of the .257 Wea. mag. The best way to describe the .25 Gibbs is an improved .25 AI.

No, the barrel would not have to be set back for the .257 Wea. Mag as it will clean up the 25-06 chamber. Yes, the bolt face would have to be opened up to the magnum bolt face to accept the .257 Wea mag. While I was having the bolt face opened up, I would do myself a favor and also have a Sako type extractor put in. It will add about $75 to the job, but will save you some time, trouble, and expense somewhere down the road.