I changed out my rims for a 18" front and 16" rear.
They are HD Blade Custom Wheels.
I decided to go with the 18/16 combo because I just thought it might ride better
(more rubber for cushion).
On second thought I should have done 18 on the rear so you could see it better.
But I stayed with the stock size, just because I wasn't sure on the ride.
I know the 21 front and 18 rear set up look even sweeter too.
You can never really see the rear wheel that good anyway.
I've been thinking about the same combo with either a shorter wall front tire or changing the fork springs in order to keep the bike level, etc.
I gotta tell you that i think imho that your set up looks as good if not better than the 21/18 set up nice stance & i bet you did not have to make any fender mods - is that correct?