A few comments.
First, I would point out for all those worried about the rush to test that based on Eladrin's comments, it appears that this is being put on to Lammania along side but not part of Update 5 for testing. Please clarify if I am wrong.
Next, I think that the change to offhand swing percentages is a good change as long as they can be balanced correctly. While this is not PnP, it is (supposedly) based on the d20 3.5 ruleset, in which a character with BAB of +16 and TWF,ITWF,GTWF would get 4 main hand attacks and 3 offhand attacks per round. Translated to this new system, a vanilla full BAB character with all three feats should have a 75% chance to proc an offhand attack, rather than the 55% Eladrin has shown. This could effectively push Tempest III rangers and Wind IV monks to 100% if done correctly, leaving the two groups who should be spinning balls of hurt as spinning balls of hurt.
I also feel it is a bad comparison to pick the acknowledged top end overpowered eSOS as a THF comparison to TWF rather than greensteel as 1)it is not nearly as common as greensteel weapons, and 2)it can only be used at lvl 20 whereas greensteel THF is available at level 12ish(?). In addition, using frenzied berserker as a means to justify the power of THF is also somewhat flawed as it is not available to a large number of THF builds. Arguably, TWF outpaces THF from level 12 to level 19 (18 in the case of FBs) when greensteel weapons are available. So I think a little balancing can be tolerated.