The idea that top Chinese fighters can't compete with top Thai fighters has been proven to be BS since 2009. China has consistently beaten top Thai fighters in almost every exchange bout losing far less than they win. The thing is, Chinese fighters constantly learn what their Thai counterparts could do while the Thai fighters are slower to pick up what the Sanda fighters could do. Some did learn Sanda throws and trips and how to defend against them, but are still unfamiliar with the sidekicks.
The idea that Simon Marcus isn't a top level fighter is also BS. Lumpini champs isn't invincible and guys like Kem lost to Arvin, who lost to Marcus. Sanda fighters like Zhang Kaiyin, Fang Bian, Bateer, has every potential to beat Thai fighters either in unified rules or kickboxing.
I have little doubt k1 champs like Buakaw would defeat Saenchai in kickboxing.
Under Thai rules obviously the Chinese won't compete, just as under Sanda rules, the Thais cannot compete. What is the point? The best rule that incorporates the weapons of both sports are the newer unified rules, and the Sanda fighters still won more often than not.