Okay, I just got off the phone with Jack with PI. He said they do still sell the lock up assist tube kit, now here's the but... You have to use an AC Delco/GM 700r4 filter with assist tube, not one from the chain stores unless it's an AC. Reason is the cheapy ones can suck itself closed restricting the fluid flow and lowering pressure. So if you use the assist tube you must use an original GM/ AC Delco 700r4 filter. If you don't plan to run a GM 700 filter, then DO NOT run the tube kit.
I also asked about not using the assist tube kit and he said that not using the assist tube can cause lock up not to be as good ( the clutches can't lock together as good ) and can cause the clutch discs ( in the converter ) to glaze when used at WOT lock up. Prolonged use like this will ruin the clutches in the converter and it will eventually fail.