I don't agree with his arbitrary 6% rule, either, but much less improvement than that requires the Ackley owner to have more in mind than just velocity when rechambering. That's a good thing. I've never had to trim Ackley Improved cases. I believe the Ackley chamber to be easier on barrel throats. If someone can get the same or better velocities as the standard cartridge, but do it in a shorter, handier barrel length with the Ackley chambering, he's gained a more useful hunting tool. And it does make for a sexier looking cartridge, doesn't it?
A more important consideration, though, is whether or not one is having a barrel rechambered to the Ackley configuration or building a rifle from scratch for an Ackley cartridge. Rechambering requires the owner to expend precious funds, only to have to reuse the same factory barrel, which may not be a good thing. The owner also winds up with an extra set of dies. However, if someone is having a rifle built, it really doesn't matter what chambering is chosen. The barrel is chambered, headspaced, and installed only once. Money is spent on only one set of dies. And if the owner is like me, he doesn't get rid of rifles once he acquires them, and has something not everybody else on the block has (always good for deer camp conversation).
