One of the pieces of common wisdom regarding the taper is that the "miles are already in the bank" and there isn't anything you can do in those last 3 weeks to improve your fitness, you can only screw yourself by doing too much.
But this doesn't jibe with other advice that is given, such as the long runs will get progressively easier to accomplish (i.e. if you do a 20 miler on week 1 and another 20 on week 3 the second one will feel easier). Same thing with speedwork. It seems to me that many aspects of training assume that you can make significant gains in 3 weeks.
So what gives? Can you make gains in the final weeks but that is offset by being tired?