there are other things to take into account. if the guy in front has very low downforce he will have greater straight line speed than a high downforce car.
if you are running high downforce and slipstream him, you would still catch (due to sliptstream) but the effect wouldn't be as great - i believe (altho i'm not certain about this)
gears are certainly crucial. if your gears aren't right, you could be hitting the limiter and not be able to go faster. today alonso ran a high 7th gear to allow him higher speed in the slipstream.
in a nutshell, if you had equal cars to the guy in front, you would enjoy a great slipstream, but if you are lacking in straightline speed, the slipstream won't seem so great.