|Chey and Boris Linden Attempt to Solve the Mystery of the Inflating Prim Count|
JIRA is Linden Lab's way of tracking problems and erratic behavior with the software of Second Life. I have to say the response to my case was prompt and effective.
My JIRA was BUG-6547, Setting Prim Faces to Alpha Masking Dramatically Inflates Land Impact When in Prim Physics Shape. It was soon referred to another, earlier JIRA, MATBUG-13, Alpha Masking Can Dramatically Inflate Physics Land Impact. It seems increased land impact is expected behavior when alpha masking is used, even when the object is set to Prim shape.
It seems to me this opens a can of worms. Until now a 1:1 Prim/Land Impact ratio has prevailed for objects set to Prim physics shape. Not it seems we'll need to check dto make sure this is true-- every time we change anything in the Edit window.