If you are serious about actually intending to shoot out to 500 meters, a fast twist barrel would seem to be the most reasonable way to go. I have never shot light bullets from my 1-8 twist, but 69 grain bullets work really well. My personal favorite is the 80 grain bullets.
I use Reloader 22 because of the superb consistency of the powder, but there is no need to go with the Ack. Imp., even with that slow of a powder. Something as slow as Reloader 25 is too slow for both case designs, so there is no velocity advantage unless you can find a suitable powder that fills up the AI case.
One possibility to consider with the AI might be to get the barrel with the standard 22-250, try it for a while, and have the chamber reamed out and set back to AI if the bit of additional case capacity will actually gain anything.
One downside of going with a wildcat that requires foreforming is barrel life. By needing to fireform 200 cases you have reduced the barrel life by that amount without ever getting into the AI potential. In something that has long barrel life, that would not be a real concern but calibers that are reputed to be barrel burners can have a significant effect.
If you do decide to go with a 1-12 or 1-14 twist you will be going with a known and well tested combination.
