I have an article from '96 about the Ackley Improveds. I'll see if I can remember to look up what it said about the .308 Improved tonight. If I recall correctly, I think the big improvement was case life, but I'll check. Also, if you want to search for Sensop on TFL (I think his email changed), he knows a lot about the AI's.
Personally, I'm gonna have George Stringer build me a "Cub Scout" in .250/3000 Ackley Improved.