Reaction time doesn't effect your ET because the clock doesn't start on Green, it starts when your front tire breaks the light beam in front of it.
a better reaction time can often win a race though, despite having a slower ET.
I've beat SRT8's, SRT10's and WRX STi's (all mid-low 13 second cars)
by catching them sleeping at the tree by up to 5 tenths.
WRX left lane 0.508 R/T, me right lane .036 R/T
By the time he caught my rear bumper at the finish line at 13.528
I've already crossed it at 13.928 because I left him at the tree by almost half a second
note the bottom of the slip, right lane 1st by .0724