Taller tire will slow reaction times and give quicker et's as said because of more roll out. Basically you get a longer run before you break the beams so you are already moving more at the start of the clocks. Or with the taller tire you can roll in deeper so that you are further down the track and slightly ahead of the other guy at the start.
All of this is small amounts though, you are not going to pick up a couple of tenths with a 2 inch taller tire. This is one of the reasons the Top Fuel guys went back the the 17 inch tires from the short airplane tires years back. Going to a shorter tire is just the reverse of all of this.
Curtis