Grinding will (generally) provide a more consistent smoking medium, so basically your herb should burn at a more constant rate and should allow air to pass through evenly which means the vapourization of THC would be greater overall.

Think of it as more efficient burning and processing - just like when you chew food

To be honest I doubt you will find many studies with decent information - a lot of them have been conducted with bias (like the chimps smoking weed - basically being oxygen deprived and calling it brain damage from weed - meh) so it would be tricky regardless.

What is interesting is the difference between smoking methods and vapourization - there are some good Gas chromo figures there

Source:
Volcano Vaporizer Study

Also available (full study) as a PDF here:
http://www.canorml.org/healthfacts/j...ingervapor.pdf

They report up to 61% THC extraction with one run of the vapourizer - obviously this would be higher if the weed was re-vaped again, as an Volcano owner will attest!