Depends on a lot of variables. Usually more SLR will make the car quicker in 60 to a point, as long as it doesn't spin and the suspension is optimized. Smaller drop at the gear change will help a smallerpeakier engine combo. SLR can be be too much or not enough and make the car a turd. When it's right, it makes the car so much better. Ive gone as quick with a 1.98 as I did with a 2.45, but the suspension was set up differently as well. And it would do that everytime - not hook 3 times and turn the tire the 4th. With a 1.76 it was a turd.

MPH shouldn't be effected unless this is 1/8 mile and the combo is way out of whack.