Does 10.7.x now include dtrace ( tracing utility ).
If you try the handbrake on both systems with dtrace running it should show you exactly the calls that are being made and allow one to make some educated guesses to possibly backup your theory.
I think it's Geekbench.
However it's kind of weird because the QTKit framework lists the systems.plist file and the MacPro 5,1 ( mid 2010 model ) is missing.
<string>iMac4,1</string>
<string>iMac4,2</string>
<string>iMac5,1</string>
<string>iMac5,2</string>
<string>iMac6,1</string>
<string>iMac7,1</string>
<string>iMac8,1</string>
<string>iMac9,1</string>
<string>iMac10,1</string>
<string>iMac11,1</string>
<string>iMac11,2</string>
<string>iMac11,3</string>
<string>Macmini1,1</string>
<string>Macmini2,1</string>
<string>Macmini3,1</string>
<string>Macmini4,1</string>
<string>MacPro1,1</string>
<string>MacPro2,1</string>
<string>MacPro3,1</string>
<string>MacPro4,1</string>
<string>MacBook1,1</string>
<string>MacBook2,1</string>
<string>MacBook3,1</string>
<string>MacBook4,1</string>
<string>MacBook5,1</string>
<string>MacBook5,2</string>
<string>MacBook6,1</string>
<string>MacBook7,1</string>
<string>MacBookAir1,1</string>
<string>MacBookAir2,1</string>
<string>MacBookPro1,1</string>
<string>MacBookPro1,2</string>
<string>MacBookPro2,1</string>
<string>MacBookPro2,2</string>
<string>MacBookPro3,1</string>
<string>MacBookPro3,2</string>
<string>MacBookPro4,1</string>
<string>MacBookPro5,1</string>
<string>MacBookPro5,2</string>
<string>MacBookPro5,3</string>
<string>MacBookPro5,4</string>
<string>MacBookPro5,5</string>
<string>MacBookPro6,1</string>
<string>MacBookPro6,2</string>
<string>MacBookPro7,1</string>