Does anyone have any expearience using the mount branding stone? If so do you think it is worth getting to help your mount getting skills. It's on sale now and if it is worth it I may buy a few to use it on the pure white T8 horses that start out with like 136 speed.

It's all subjective. You won't know decisively whether it helped or not because it's still RNG. Some would say no and would also say that horse costume sets are not worthing using either. Others would say yes that both are. I can say that branding stones are very nice for fishing rods and if we can extrapolate the strength of those to be comparative to the horse branding stones it would seem to me that are worth using if you do not mind spending money and want the best chance for skill gain.