I can lend a bit of advice on transmission tuning as I have had LT1/4L60Es for a long time.

1. The 4L60E has no built-in pressure sensor to provide feedback to the PCM, so all it can do is gauge shifts based on time. Just like you bought a wideband, buy a transmission pressure gauge and drive around for a few weeks with the gauge strapped to your windshield wiper like a hood tach so you can get an idea of what it's really doing pressure-wise. Ebay seller cmbclalb sells a good one that I've used for a long time. A healthy 4L60E will have around 70psi at idle in Park, Neutral or Drive, and climb to around 150psi in reverse. You should see over 200psi at WOT at some point in 1st gear.
2. Think of line pressure tuning as two separate regions; during shifts, and between shifts. If the trans is not slipping in gear, then leave the line pressure tables alone and ONLY reduce the "Shift time (sec) vs. TPS vs. shift." In the last LT1/4L60E I tuned, the WOT shifts were perfect at 0.600 sec for 1-2 and 0.875 sec for 2-3. Leave the 3-4 shift at 0.00 seconds because the 4th gear apply piston is small and needs no softening. That 1-2 shift value gave me just a slight chirp from the rear tires. I tried 0.400 sec but it was too harsh and barked the tires. 0.800 sec wasn't firm enough and was actually cutting pressure during the shift according to Datamaster.
3. The 4L60E is a bit of a hybrid transmission in that the shift solenoids command the START of the shifts, but the actual shifting is still completed by hydraulic valves, unlike something like the Chrysler 41TE where the solenoids directly fill the hydraulic circuits with no additional valves. Sometimes these shifts take a long time to complete, so if you're hitting the rev limiter and you have sufficient line pressure during the shifts, you need to reduce the shift MPH to get the shift to complete on time. A looser converter, numerically lower rear axle, more engine torque, and more vehicle weight will all exacerbate this late shift behavior.
4. Shifting harder IS shifting faster, and vice versa. It's really a balance point. For modded vehicles, the only way to get your 4L60E shifting perfect assuming it's in good mechanical health is to dial it in over the course of a datalogging/tuning session, or in my case, a road trip.
5. No need to up line pressure with a looser converter unless you think it's slipping. The easiest way to do this is to reduce the "maximum line pressure" scalar 5-10% which will give you 5-10% more pressure across the board at part throttle, but it will NOT give you any more at WOT once the sum of the line pressure tables equals or exceeds the scalar.