Edit:
Thanks for the massive amount of feedback all.
We're currently thinking of adjusting the numbers to:
Code:
Doublestrike Bonus Main hand Off hand
No feats 0 20% 100% 20%
TWF 0 +20% 100% 40%
ITWF 0 +20% 100% 60%
GTWF 0 +20% 100% 80%
STWF 0 +20% 100% 100%
Tempest I 0 +10% 100% 90%
Tempest II 0 +10% 100% 100%
Tempest III +5%* 0 105% 100%
Wind IV +10% 0 110% 80%
Zeal +10% 0 110% 80%
Alacrity +10% 0 110% 80%
* Only when wielding two weapons.
This set switches Wind Stance and Tempest III to doublestrike bonuses, increases the benefits of the TWF feat chain, and adds an additional feat for high BAB characters.
All of the bottom rows assume that the person has GTWF, except for the STWF row.