Ok I thought someone would have something to say about this by now so I'll chip in my 0.01 cents (cos my knowledge is really limited).
While I was in Thailand I visited plenty of temples. I went every Sunday for a while. I've been blessed by a monk before a fight where I went on to have my worst loss. I also went to a cermoney where they blessed a gym I was training at to suffer a bad infection that same day that completely floored me for 2 weeks.
So I think spirituality and buddhist principles don't have much effect on any outcome in muay thai.
That said, Thailand is one of the only countries that was never colonized so they do have a lot of superstitions that go back thousands of years. Plenty of pracitices in muay thai are directly related to this but whether it didn't help me because I'm a terrible buddhist or what I'll never know.
Sak Yant tattoos are still popular with thai boxers (and a large portion of the thai population), people still visit temples every Sunday, they are in general a very religious race (at least in principles).