With the 1/2 wave you should be good as they aren't as ground dependent, but with a 1/4 wave that mount isn't going to perform very well.
The powder coat isolates it from the rack and the rack will most likely be isolated from the body of the vehicle by paint which means that your coax will be trying to radiate.
I run a trunk lid type mount on the side of the rear gate on my X with the coax above the head liner to the front.
I had to drill extra spots to get the clocking the way I wanted it and when I got it, I disassembled it and removed all the paint in the pivot areas and put a coating of copper antiseize in them when I reassembled it. It does OK with grounding since the set screws that hold it on penetrate the paint to the metal. I used antiseize on each of them as well and have had no issues with it as a Ham, GMRS or CB mount. It's dedicated to Ham right now with a $13 Tram dual band NMO from Amazon as well.
