Anyone ever use valve/lash tuning?

I presume you are talking about solid lifters. Effect is very limited. If you go much looser than the stated lash, you risk valve train damage/failure because you move off the lash ramp of the lobe onto the opening ramp.
Going tighter does not cause a problem as long as not so tight that valves are not seating.
Changing lash has more effect on idle quality/vacuum & very little effect on shifting the power curve.