I will outline why TaeKwondo, no matter contact level, or even rules, breaks down in a Muay Thai ring against a skilled Thaistylist. This despite the fact that all strikes in Muay Thai are also in TaeKwondo! My reasons may surprise you.
1. Footwork. TKDoins are often renowned for their speedy and light footwork. However, being light on your feet in a Thaiboxing ring also carries the disadvantage of being easier to knock OFF your feet. Thaiboxers in comparision are more flat footed but rock solid to the ground. Getting put on your butt repeatedly will wear and tear on you. And this will happen even more easily with one foot up in the air kicking..
2. The kicks. This ties in to the point above. TaeKwondo kicks are snappy and quick of the feet, but makes one liable to get pushed off balance in a rule set in which you can actually grabb and push down kicks. Stability will once again be a major question mark. This is demoralising in the long run and, the kicks you perfected in the dojang won't get you as many points (or KO'S) as you might have previously though..
3. The hands. To break a skilled Muay Thai fighters defence in the ring you will need to soften him/her up with boxing, and/ or have a boxing defence to punches. Kicking alone will not get the job done against a legit guy(especially not concidering 1 and 2). TKD guys are not drilled in either offense or defence to the level required.
4. No clinch. This can be modified in theory, but I will list it here given that most clubs neglect it.
5. Lack of Continious fighting. TKD is geared towards one strike, one victory philosophy. Even WTF rules that allow full contact break after making contact. It will be an adjustment for a TKD fighter simply fight on and is definately puts him/her in a slight psychological disadvantage, compared to the thaifighter.
Feel free to comment and disagree on any of the points!