I'm eagerly awaiting the response from the ARP tech.
As a side note, I recall reading that thread depths deeper than the diameter of the shank are useless, as no tension is applied to the later threads. I never believed it, but I'll try to find it.
Here it is:
Thread Engagement Issue
"When a bolt is fully engaged into a nut or when a screw has a depth of engagement at least equal to the diameter of the screw, the thread engagement is adequae [sic]. Special situations arise when the bolt is not fully engaged into the nut or when the screw is engaged into a much weaker material. Sufficient design margin against failure by thread stripping should be provided."
http://www.pdhonline.org/courses/s149/Bolt_Analysis.htm
It seems to just be a rule of thumb, and doesn't even hold up when the receiving material is of lesser strength than the bolt (which is the case here). Nevermind...