There are existing studies which show that people react faster when startled than when not. There's a nifty little online test to check your own response times: http://www.mathsisfun.com/games/reaction-time.html
As far as I know this is the first test to show that startle response extends beyond reaction time, it's an interesting piece of data. It's worth noting that it's possible to force yourself into a state of mind similar to being startled, which may very well eliminate the advantage to "shooting" second.
That game is very telling. Try it at "arms length", then again with your face right up to the screen. See if you don't do much better when the dot fills more of your visual range. Speculation: a startle reflex depends upon the magnitude of the nervous signal, which is larger when more retinal cells fire.
As far as I know this is the first test to show that startle response extends beyond reaction time, it's an interesting piece of data. It's worth noting that it's possible to force yourself into a state of mind similar to being startled, which may very well eliminate the advantage to "shooting" second.