I used tail extensions in APHA shows when I was showing, for both Bali and Foxy. It made them both look a lot better, in my opinion. I have never used a forelock one, and personally I think that's a little overkill... HOWEVER, if ever a horse needed one it's Bali- he has a little puff of forelock and that's it hahaha he looks like a goober.
Also- I wanted to add, I always used black for Foxy who's tail was mainly white with some black in it, I just thought it made it look fuller... and Bali's was flaxen to bring out his natural little bit of flaxen he has mixed in with his typical reddish tail. I think it looks good to mix in a color in their tail you wish they had more of, that way it's not just all one giant solid color- if your horse has a mixed tail as you mentioned :)