You'll gain right at a 5% increase in powder capacity which translates to perhaps a 1.5% increase in velocity at same pressures. Assuming 3000 fps for the parent case that gives 45 fps more, hardly worth writing home about. Rate of throat erosion will be reduced by around 10% which is good considering the .243 eats throats. You will also rarely if ever trim cases. I trimmed a batch of cases right after fire forming and over 25 firings they never needed trimming again. When I finally tossed them at 25x they had only gained around .005-.006" in length.
Getting back to the velocity increase. Lots of AI shooters claim greatly increased velocities because they load to higher pressures. In the .243 I think that might be somewhat valid. The .243 shows a wider variation in pressures in a given string so it's max pressure was lowered some time ago. The 40 degree shoulder of the AI tames that bad habit so one could load back up to the original pressures. Doing that combined with the velocity increase provided by increased case capacity could net you maybe 75-100 fps more. That's just theoretical discussion which I wouldn't personally condone unless you know the pressure specs of the original loading, have very good pressure testing equipment and know how to use it.
But the increased throat life and lack of trimming are the main benefits for the .243 AI.
Now all that said, I've had two barrels so chambered and if you like to mess around with such things they're fun and something interesting to work with. I certainly learned some things as a hobbyist so they were worthwhile projects. If you just want practicality get a 6mm Creedmoor and be done with it.
