I owned both the 70-200m.ii and the 70-300L and now the 100-400L... I still regret selling the 70-300L... it is visibly superior to the nonL version. The only reason I sold it is that it does not work with the 2x Canon tele's...
Using the 70-300L with a 2x is maybe not such a good idea anyway? I don't have the Kenko 2x, but the 1.4x and it's working fine, but I wouldn't want to push it any further than that because of loosing af completely and iq breakdown.
- IS is not to the same standard and the VC does not offer a tripod detection
... but not that Canon would include a tripod ring by default, it's only a $1300 lens - thanks, Canon! I just discovered from some long-term exposure shots at dawn that there's really no getting around a collar.