This topic came up on one of the FB car groups I read, the question was why a good majority of Japanese shops don't paint their motors and accessories. I worked in a couple of shops in Japan and the answer was for a couple of reasons.

1) Paint interferes with grounds.
2) If the paint is thick between bolted parts, it can crack, chip and fall away making the parts loose especially if mounting bosses are painted. . If the car is going racing, they won't paint anything , they will polish a lot of parts though.

One pretty famous tuner there also swears that painted motors run hotter according to what he saw on his engine dyno.