I have just bought the book 'rock discipline' by petrucci, because the one al di meola had written is out of print, and there's a lot of exercices you might want to check out. As for the rate of improvement, is will be tremendous at the start, then you'll have to stagnate a little, then you'll go up again (or over and over..) Just know that's there's a speed limit that's determined by your muscles... (Check out the fastest guitarist thread to check some monsters out)