You will no doubt get better at the things you practice most....that's pretty obvious by the concept of practicing. If you practice something, then you'll get good at it. Are you just pointing that out?

I wouldn't go as far as to say "whatever you practice is exactly what you'll master" though. It is up to you to take what you practice and apply it to other areas. That is what's going to make you a better musician. If you practice scales all day, it isn't worth much if at the end of the day you ONLY know scales.