Using pistol primers in the Hornet has been around for a long time, probably before Mic was born. They weren't supposed to work, according to some companies and "experts," because of piercing and blanking. But then the last Speer Manual (#14 I believe) gave pistol primers the seal of approval, using them for all their Hornet loads.
I've NEVER had any piercing/blanking problem with CCI 450's in any Hornet load, whether the .22 Hornet, .17 Hornet or K-Hornet. For that matter, have never had any problem with small-pistol primers either, but as far as I can recall have only tried CCI 500's.
What I've found, not just with various Hornets but other small centerfires, is different primers can make far more difference than in larger cartridges. This is partly because small rifle (and small pistol) primers vary far more than large rifle primers, both in cup material/thickness and brisance, but the small capacity probably has something to do with it too.
Early last year I measured the H2O capacity of fired Winchester cases from my .22 Hornet and K-Hornet rifles, using 40-grain Tipped Varmageddons seated to 1.79" overall length, and the difference was a 10.66% increase in the K-Hornet--close to what you've heard. Applying the 4-to-1 Rule suggests a 2.66% potential increase in velocity, about 80 fps in a 3000 fps load. Of course that would vary somewhat due to other factors as well.
