Well, I suspect the seal is essentially trashed, but is the recoil greater or less than it was before you ground your seal?
I would refrain from shooting it if it were mine. The piston requires a cushion of air in front of it at the end of the stroke to prevent it from slamming into the end of the compression tube. The undersized piston seal will allow that air to blow past it and not build the pressure it needs. That is why the shot cycle feels different. Repeated shots without this cushion could lead to damage. Or maybe not, but that's an awful nice rifle to take risks with. It's well built, but it's not designed to shoot with an undersized seal.Good luck!