Nope its a direct fit, you would just have to take it off. And yeah the numbers are probably going to be the top speed, actually it may be a little higher than that, but the lower your top speed the more acceleration your gonna have. Usually the best ratio is one where you run out of RPM's right before you have to brake into the next turn, if you run out of RPM's midway down the straight your losing alot of time, but if you don't get your RPM's near max at the straight then you actually may be going slower, or the same but you will definitly loose time on the turns. I would recommend the 17T anything less than that your car may not be fast enough on the straight.