When I do the math on SA vs. Climactic Flourish, I get that SA is better pretty much all the time. I know I've done pretty significant Climactic Flourish'd Rudra's to stuff. So you wouldn't actually need a DD buff to make the idea I propose
here viable. The problem is that you guys don't often get a chance to use the mechanics of your jobs, SA and TA. That problem would be taken care of by changing the TH system to what I described.
Fringe benefits:
1) As content gets outdated, THF max damage rises and drop rates effectively are increased.
2) Having relic/mythic/empyrean weapons primarily boost your single-hit SA/TA WS damage, which would also boost your Treasure Hunter through this system. Mandau > Thief's Knife. Heck, Oneiros Knife or X's Knife > Thief's Knife offhand.
3) People start playing with playstyle focused around THF mechanics. They actually set up SATAWS situations, because that gets the highest THF damage and the highest drop rate.
4) As long as you make it only spike damage that matters, THF doesn't become the only melee. Dragoon, for instance, would get a sideways HNM buff from this because Angon would be useful for improving THF damage. People will do anything in the pursuit of loot.