While shift firmness can be effected by increasing the line pressure electronically (force motor), and the shift solenoids time gear to gear shift points, and these things can be changed with tuning changes, there are still many mechanical things that also effect the "personality" of the 4L60-E. Accumulator springs can be replaced with stiffer springs or be broken, servo area changes, separator plate hole sizes, broken valve body springs, and how well the valves seal and don't stick, or have leaking end plugs. Clutch packs, and how good the apply pistons lip seals hold line pressure, pump condition,O-rings in many places, stuck checkballs.......... the list goes on. Just thought I would throw this out there because while a transmission that has no other issues, that has been assembled correctly, and is in overall good running condition, then yes...the torque converter lock up depends on the signals the TCC and PWM solenoids receive . But there are mechanical things that predictable converter lock up depends on too, and if they fail, even if the TCC, PWM, PCM, connections, and tune are perfect, the converter will not perform as expected.