The best form of thaiboxing is dutch thaiboxing -which is a meld of thai style thaiboxing, english boxing and karate (predominantly kyokushin karate).
Japanese kickboxing has strong roots to both thaiboxing and kyokushin.
Yes all kyokushin fighters in k-1 have complimented with english boxing training, but so have all thaiboxers , even the Thai ones (yes, even Buakaw).
And more effective for what?
Kyokushin fighters need more adaptions in preparation for the k-1 ring than traditional thaiboxers due to the traditional knockdown rules that 1. dont use a ring. (=drastically different ringcraft). 2. Do not use gloves. (striking and blocking with gloves and without gloves are very different). 3 do not allow punches to the head. (yes it is a realism flaw and is the result of safety regulations back in the 60ies -all sports have realism flaws, live with it).
So more effective for what? 1 and 2 would not be an issue in say a streetfight. 3 would be a issue, but MT have realism flaws due to rules too.
But kyokushin and knockdown fighters do not fight only with those rules as there are several types of competitions (and kyokushin is not only competition -there are lots of stuff taught that are banned in all sport rules) for kyokushin type fighters who want to hit the head aswell. Shinken shobu, shinkarate/gloved karate, not to mention all those who fight in kickboxing.
Another problem is who are fighting. The kyokushin fighters who are in K-1 are generally "official" flagship sent there by the Matsui group of kyokushin (kyokushin fighters from other groups are not welcome due to political and business deal reasons). And the matsui group "flagships" tend to be fighters who have spent their entire fighting career specializing to fight under formal knockdown rules.
In MMA there are not that many kyokushin guys, but truthfully, there are not that many thaiboxing based fighters either. Thaiboxing in MMA is popular as standup, but it is seldom a fighters original art. And thaiboxing is admittedly faster to get into than kyokushin karate with a lot less formal stuff that established fighters who just want to compliment their fighting need to plow through to get to the good stuff.