I'm not trying to be a smart ass here, I'm really not. But what TKD fighters do you know who are at the top level of MMA? I've heard of a few fighters who trained TKD as kids, and one fighter who said he wasted his time earning a TKD black belt, but that's the extent of my knowledge.
kung lee trains Kung fu and TKD Doesn't he.
I took tkd for a while (Because it was free at my collage) I think TKD could be a huge help in MMA but its only one part. Like everyone else you have to be well rounded you have to have wrestling kickboxing and jiujitsu to be successful. But if you can master some of the basic kicks in TKD like the front and side It could be very help full.
Like bruce lee said the best style is no style at all. And when he made JKD he put the sidekick of TKD the holds of BJJ the throws of judo knees of MT the takedowns of wrestling and the punches of western boxing and kung fu.
if you would learn to throw kicks like bruce I am sure that would help alot.
"The fat man flies at midnight"