I'm much closer to a beginning pistol shooter than you guys in here - with that said:
When I got to learn from Gabe earlier this summer, he did two things with my grip. He had me take a little bit of the bottom of the heel of my right palm off the grip, in order to cement the whole left support hand palm onto the grip. Then, he had me "torque" my left hand up into the weapon light rail area.
This tightened up my groups and took them from consistent left or high left, to centered. I have adopted this for all my shooting and am learning to change my draw so that I get that grip by the time I'm extended.
These changes made several magnitudes more of a before/after difference than any changes I can make in grip pressure, two handed or single handed.