When I had a big dollar 4l60e done, it would shift just nice and firm at low throttle and would bark the tires at WOT. If the trans is barking the tires at low throttle angles, something isnt right IMO.

I remember when when I had a MIL and the PCM was commanding 100% line pressure, the trans would bark the tires with ANY throttle at all.

A proper shift kit should shift softer at low throttle angles and harder at WOT.

Stock trans, you can raise line pressure via the tune.
Modified trans, allready raises line pressure mechanically, so a near stock line pressure tune should be used.

peace
Hog