#1
I found a softwer that speeds up mp3, so I got an idea to speed up songs I know well and play along to 115%. Only play rhythm part though. I tried with metallica's end of the line, seek and destroy and that was just your life, and megadeth's never walk alone. Is this kind of practice good?
#2
Probably won't hurt, but it would be better to play to a metronome and with no other guitars in the mix, just so you can hear if you're playing in time with good articulation and tone, etc.
#3
It's great if you're technique is still in complete control. Though if you have any problems with relaxing while playing and you play the whole song through then it might create too much tension and your technique will suffer. But in small segments and with concentration, I think that practicing already learned songs to even higher speeds is a great speed/technique builder that is so often overlooked.
Though it seems that your asking about playing the whole thing? If so, then I'd say it's not a good way to get better. You improve with repetition and the smaller the segment, the more repetitions you'll be able to get to.