Interesting question. Machida's TDD for the most part has looked sublime to say the least. The way he made Tito look like he didn't know how to shoot was a masterclass in sprawl and brawl..or rather, clinch disengage and pick apart. Thing with Machida is he has good TDD defence from all areas. Ortiz isn't the best wrestler, but he's a strong guy with some technique and even when he did get his hands on Machida, Machida just completely outclinched him to either release himself or get a better positiion.
I suspect his background in sumo helps with this.
I think his TDD will prove to be more or less as good as we expect it to be come the Couture fight. Randy hasn't got the best shot these days so shooting on Machida will be a mistake because of his fast reflexes and ability to guage distance. If it was somebody like Rashad or a wrestler with that type of explosion with their takedowns then I can see that causing Machida problems, but Couture isn't the right age and doesn't have that explosive power anymore. And the clinch scenario will play out in a similar way to Randy vs Brandon Vera except Machida will be more aggressive and probably make Randy pay everytime he tries to bodylock him.