In all honesty a solid rear rim may catch a bit more wind regardless of the fact that you are on a bagger than any other sort of rim. Were it my bike I would do something different. I realize that you do not see much of the rear rim but I see it as a waste of money.
Either powder or chrome the stock wheel or get s et of wheels. that is what I would do. Of course not everyone does things the same but I hate to see money wasted.
You can find them but they're not all that much cheaper than a custom wheel. HogPro makes one for around $1100 and Yaffe has one for about $800. I considered going that route but when I saw the prices I just went with the matched wheel.
Here is a picture of an 18" front and 16" rear. Although I am pretty sure you can not get a 16 agitator. That was the wheel that I was looking at. And then I got the Blade wheels, so that I could match.