Only thing I've seen that can adjust polyphonic material like guitar parts is Celemony's Melodyne Editor and above which has a MIDI output. How well this works, I don't know, but on audio, this thing is close to miraculous (if the audio is clear enough), and it is capable of generating a MIDI file with notes and velocities correct, if the manual is to be believed.
http://www.celemony.com/cms/index.php?id=715&L=1.%253FL%253D1.%253FL%253D1.%253FL%253D1

An utterly groundbreaking product (but not cheap!)