From what you've described , I believe that buck is still chasing tail out there.

I wouldn't jump to the conclusion that a mechanical head was the sole reason behind the problem. Could have been the type of mech head. There are other bad experience stories out there concerning the snyper. Fixed blade broadhead users have had terrible penetration on leg bones as well. If you were to tell me that last year you shot a fixed blade head (with your same setup) thru a shoulder bone with no problems , I'd cross that type of head off my list of things to try.
I will say it's pretty bold for NAP to say their spitfires routinely shoot thru leg bones , but I can vouche for the fact that with the right setup , it's possible.
In the following link , the bottom picture on the right shows the wound on the buck I shot this year. The 100gn NAP spitfire blew thru his leg and stopped in the leg on the opposite side. But , I should add though , with my current setup , I'd be willing to bet that if I would have had my muzzys on , I would have blown out the opposite leg as well.http://community.webshots.com/user/1bowfanatic