For me, middle finger pressure would take away speed and give the pitch more drop due to velocity loss and arm slot. If I used index pressure, I'd get more of a faster flatter slider with later and less dramatic drop.
In my experience, the more you focus on one finger or the other with the slider, the more differential there is with the fastball speed and rotation. I used to throw curves and sliders from various release points now and then to keep the batter honest and when I was throwing a curve in a curve count, I'd give it a wrinkle so it wasn't the same as the last time they saw a curve. Stuff like that makes the difference between throwing and pitching.
I was RHP, so against LHH I would throw the high hard slider running from inner 1/3 to hands, go away with a two seamer cutting across the outer edge and off the plate then follow it with a soft slider or curve down and in breaking toward their back foot. Then a high and away 4 seamer then a back door curve. The final 3 pitches were all out pitches in that sequence. Give them nothing they want, but give them something that makes them think you did.