I hope it's okay to mention, as it is not a traditional classical music piece, but I am attempting to learn the acoustic version of Mason Williams', "Classical Gas". I'm using an android app called Music Speed Changer which allows me to adjust the tempo of the original recording without changing the pitch. It has definitely helped me to get to 70% of normal speed.
I'm just returning to playing guitar and my approach to learning this piece has been a learning experience too. Break the song into parts, identify troublesome spots and keep playing those spots until muscle memory kicks in and now I'm practicing not using the TAB and playing it all the way through completely from memory. By doing this last bit, I am amazed at how much more I am able to appreciate how the song was constructed and can "see/feel" patterns that I wasn't noticing before when I was focused on just having the TAB tell me what to do. This was a big "wow" moment for me!
Thanks for the opportunity to share! I'm new to this site and think it's fantastic!