do voice lessons improve your voice even better, like 50% better if one has a pretty nice voice to begin with?
Yes, they most definitely do. Taking voice lessons helps teach you how to properly use your abs to support your voice, instead of your throat and jaw. I take voice lessons and tension in my throat and jaw is what I have the most trouble with. It also helps with intonation and to improve your range.
I had a pretty good voice, but I started taking voice lessons this past summer, learned a few songs, and performed one at a studio class. My friend, who is a really good singer, told me that was the best she'd ever heard me sing. There was a noticable improvement in my voice, my intonation. I also learned that I was an alto, even though I had been singing first soprano in choir all year long.
Voice lessons definitely help. If you want to improve your voice, they are one of the best investments you can make.