When I played paintball, I used my middle finger to shoot. As a percussionist, this allowed a natural hand/finger movement to shoot since it replicated "stick taps". This allowed me to shoot an Autococker at 12 shots per second with a single trigger in semi-auto.
I do not shoot real firearms this way. Here are the reasons:
1. It places my index finger very close to the slide. Any misalignment can cause issues.
2. It makes it very hard to move my middle finger out of the trigger guard.
3. My support hand is in a very different spot over paintball. Since my hands overlap with a pistol, everything fits better with a normal "thumbs forward" grip.
I am not a firearms expert, but I did play in semi-pro paintball for ~10 years (NAAPSA, NPPL, PSP, etc...) and played ball for 16 years total. I am very new to shooting pistols.