So I've been testing a little with my Enhancement-Shaman on the PTR and after a while noticed that FT did less damage than on live servers - by quite a substantial amount.
Weapon of choice: Angry Dread
Totems, Buffs, etc: None
Testing Subject: Level 70 Target Dummy
Flametongue Damage PTR: 197 non crit
Flametongue Damage Live: 334 non crit
After monitoring closely my procs and resulting changes in spell power, I can assert with a high degree of confidence that for some reason spell power gained from AP (via Mental Quickness) doesn't affect FT-damage in any way in the current PTR-build.
To confirm this, I dropped my FT-Totem which resulted in a damage increase of exactly 20 (144/7 = 20.5, bingo!).
Skimming through the PTR- and Shaman-forums a bit, I couldn't find any official word re: this topic, but going with Occam's razor here it *probably* is a bug, especially when you consider the fact that the devs just tinkered with FT to prevent fast weapons from scaling better than slow weapons (a very welcome change by the way).
Also, it wouldn't make any sense that Mental Quickness affects every source of spell damage except FT, and besides, it would very likely force us back to WF/WF, which according to a recent blue post, is exactly what the developers don't want.