I want to learn the song 'manhattan' of Eric Johnson, and I know, there are fast parts in it, and he has a lot more feeling than I do, but my problem is the following...

I can play as fast as he does when I improvise, but I haven't learned anything this fast before, and when I improvise, I prolly play a lot easier stuff than he does.
Well, would it be smart to learn this song, or just try something else, slower, first? Any suggestion then, I really love Eric Johson's songs.
You could learn yhe song but try it at half speed first and then a bit faster en eventually you play the whole song