Edit (6/1): In light of the changes to TWF (which impact Tempest I builds more than others) I thought I would revive the thread. The 18/2 build looks to be much better DPS-wise than the classic 12/6/2.
I recently downloaded the DPS Calculator made by Absolute-Omniscience and Yargore. I was mostly fooling around with a few builds and weapons until I started to notice a pattern:
For weapons with x3 or better Crit multiplier, Kensai III DPS >= Kensai II w/ Tempest I DPS
For weapons with x2 Crit Multiplier Kensai III DPS < Kensai II w/ Tempest I DPS
I used an 18 Fighter/2 Rogue and the Monster build as examples to evaluate the difference. With the sneak attack bonuses on one build and the favored enemy bonuses on the other, khopesh was essentially equal. For the other weapons there was a 5-10% difference in one build over the other. The calculator does not have all weapons types, so I concede that there may be some strange outliers.
I tried to be as fair as possible when comparing the builds (e.g. both have Tharnes Goggles or neither does.) Obviously real game situations will benefit one build over the other. I also don't want to gloss over the other benefits that come with 6 Ranger/2 Monk or 2 Rogue. However, I want to see people stop assuming that 6 levels of ranger makes every build better.