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.
More problems:
Currently a level 20 tempest ranger has +10% attack speed and +1 extra offhand a attack, while a level 20 fighter has just a +10% extra attack speed. So a 20 ranger should, in theory, be one-offhand-attack's worth better at TWF than a 20 fighter.
Level 20 Fighter with TWF, ITWF, GTWF, STWF =
10% double strike/100% offhand proc
Level 20 Ranger with Tempest III =
5% double strike/100% offhand proc.
As you can see, the fighter is a BETTER TWF than the ranger! This, i think, is a problem
Other comparisons -
20 Ranger Arcane Archer with STWF (1 feat investment, easily attainble with archer dex high enough to meet preq) =
0% double strike/100% offhand (Only 5% double strike behind a tempest ranger, losing only sheild bonus and to hit bonus, which not many people care about in late game anyway... but with pretty spectacular archer abilities to back them up, and no wasting 4 feats to get tempest III)
20 Rogue (Or other 3/4 BaB class) with TWF, ITWF, GTWF =
0% double strike/80% offhand proc (Fighters and rogues used to have the same offhand attack amount, fighters can now get potentially 20% more)
EDIT -
Suggestion: Abandon STWF. Let all non-ranger tempests cap their offhand proc rate at 80%. This will bring fighters back in line with rogues, will seperate Arcane Archers melee from Tempest Melee, and will make Tempest the best Two Weapon Fighters again.