my buddy did a build on a sako action, he said they have a lot more flex to the action and aren't that great of a platform for custom builds especially those with very heavy barrels.
Your buddy has no clue what he is talking about Sako is one of the better factory actions for a custom build, I have several including an L579 that the OP is asking about.
The 243AI gains you very little compared to other Ackley chamberings because you are mostly changing shoulder angle, the body has very little taper to start with.
Never really found the claim that Jim in Idaho makes of increased throat life to be valid in any of my Ackleys but less trimming for sure
As for the OP's last question, yes you can rebarrel it back to 243 or any cartridge you choose that has the mythical .473" bolt face, The L579 is a short action so you could go 22-250 to 308 and still use the magazine
