I personally have never tried that, but i do remember a while back a few people did try it and if i remember correctly, they didnt see any gains nor losses in times. I think the reasoning for that is because the computer has to adjust everytime you open and close the cutout, and while your going down the track it never actually adjusts cause it actually takes a while for it to adjust. I think thats what the verdict was, but somebody else will chime in im sure and say what the deal on that is for sure.
I guess it depends on the mods you have. Me and a buddy always open her up from the start along with the short belt to gain a couple more HP.
We both have the rcsb and we found we went slower with it closed.
depending on your mods... your only gona see little if any gain off the line and thats for less than a second or two. the rest of the run will all be top end as you'll never drop low enough to need the torque you might have lost. like turbo lag, only happens off the line. once you get into the power band... you'll never notice it