Brief TH upgrade summary:
Nynja ran a test here that seems to confirm what we were (in Engrish) told by the community reps. TH upgrade rates going from TH7 (in gear) to TH8 seem the same as the TH upgrade rate going from TH3 (in gear) to TH4.
http://www.bluegartr.com/threads/113366-TH-procs
Mojo also ran a test using Bounty Shot on Ranger to raise TH that he summarized to me in a PM. He made a character spam Bounty Shot on the same monster over and over (doesn't do damage) and eventually did get up to TH11 (with AF3+2 hands on, so TH3+8), but going from TH10 to TH11 took him over 600 shots. We don't know how potent Bounty Shot's ability to raise TH level is, but if it's comparable to SA/TA's then this would explain why you almost never see TH levels above 12, let alone TH15.