Hugh a straight line is always faster than a straight line plus something else. And an omote technique is a fast and decisive tactic against this kind of strike and if you are committed to always making a tenkan you've already eliminated some of your options.
I'll buy that the straight line is faster if you're measuring from point A to point B, but I don't think you want to do that here. I'm measuring from the time my attacker realizes I'm not going to be where he's striking to point B. If I can keep him focused on his strike by moving in, it's a very short and quick motion to turn out from under the strike. And I'm not committed to anything until I've done that.