I used to play video arcade games like Point Blank and Time Crisis and I also used to use my middle finger on the trigger, simply because I could fire much faster like that.
However when I tried to fire my Vektor CP1 under stress with my middle finger, I found that the index finger interfered in one of two ways (eventually):
1) Being on the edge of the eject port at times. Although it never caused a stoppage it was a distraction.
2) Causing unwanted friction on the slide and also distraction because the slide is textured and this movement is felt under the index finger.
So I would say on balance it is probably not a good plan for real guns. For arcades though, let it rip!
However when I tried to fire my Vektor CP1 under stress with my middle finger, I found that the index finger interfered in one of two ways (eventually):
1) Being on the edge of the eject port at times. Although it never caused a stoppage it was a distraction.
2) Causing unwanted friction on the slide and also distraction because the slide is textured and this movement is felt under the index finger.
So I would say on balance it is probably not a good plan for real guns. For arcades though, let it rip!