Thicker isn't an indication of better, it's an indication of it being just different!
What IS critical is to apply it properly which will take some reading and following the instructions along with purchasing a test tool. You really need to apply the coats (most call for 2, and may call for a thinned primer coat on some substrates that does not count) within the min/max of the specifications...if you 'wing' it, you could be way off. You need to swing by a paint store and buy a (they're inexpensive) wet-film thickness tool. That is the only way you're going to reliably determine if your coat is the proper thickness. It also requires that you understand how to use it and use it properly. It's not hard, but I've seen supposedly 'expert' people use them incorrectly. Give 5 people the same brush or roller and have them paint something, and you'll end up with 5 different coverages...some may or may not be proper. The only way to know is to measure it...you really can't tell by eye.
Exceeding the design thickness is as bad as having it too thin.
I'd cover at least as high as the showerhead with it.