The most blatant problem is a supposedly-beneficial quantity having a negative effect: increased BAB reducing your DPS. For 1wf and thf combat there is only a minor negative, where longer attack animations might have a few percentage less attacks per second. (In that case, the reduced rate is almost compensated for by the increased chance to hit AC)
It only becomes a major problem when TWF is involved, because it's TWF characters who can lose real DPS when their BAB increases to a new animation. That's because they need higher TWF feats just to stay in place, instead of to improve. Currently that only happens when a TWF guy goes from BAB 9 to 10, but if it also happens at 19 to 20 then that's a major new failure.
Uh, they already DO have an equivalent to that. +10% melee speed means they get extra attacks. If the tier1 Tempest bonus had been less than 10%, then it might be OK to give them a big bonus at tier 3.