The 10% increase in case capacity usually provides a 2-3% increase in velocity using mid-weight bullets. With the 280 vs 7mag if you are getting an 8-10% increase in velocity with a 10% powder increase you are either under-loading one or overloading the other; or using different barrel lengths; or using heavy for calibre bullets which BTW is in my opinion where the "magnums" really shine.
As for the A-Bolts .....................
I have owned 3 over the years, 2 "original" and a newer A-Bolt II. The first two I loathed, the A-Bolt II is my favorite rifle. I am waiting to get my hands on the new X-Bolt as it appears to have addressed some of the "problems" pointed out by A-Bolt owners.