I am looking for my hats as I write this. I a prior business I had they proved to be effective advertising for me. I also gifted way more than I sold. I typically gave them to girls that visited the trade shows I was at. Also all R/C car racers got one too.
I found that a darker solid color with white lettering and my logotype worked best. I'm looking at a dark grey hat with white letters for my bladesmithing. Light silver or grey Shirt with pocket and black letters for me and staff, possibility polo style. I do not plan on T-shirts for sale or give aways at this time.
Bottom line I believe in their power as a advertising tool, not a money maker.
I like the back of your design, the front looks very busy to me.