Not just pressure
Just turning up the line pressure may help a little for a little while. But as mentioned above if any clutch is slipping now, it already has an engraved headstone waiting for that final day when it is slipped one last time and can no longer hold any power. Then you are on the end of a tow truck, and the headstone is planted.
Think about the stock transmission as a garden hose that is used to provide pressure to some device at it's end. This hose came from the factory with a few leaks, these leaks were the result of poor manufacturing, some design oversights, some price reduction efforts, and sometimes poor assembly.
Lets say that device at the end of the hose needs 60 psi to operate, and the hose needs 70 psi supplied to deliver 60 psi to the device because of the leaks. This hose gets old, gets stepped on a few times, gets baked in the sun, gets frozen in the winter, so the rubber shrinks or developes more cracks and leaks, and soon there is less pressure to the end device. So the device starts to not work. If you open up the spigot to increase the pressure in the hose and thereby to the device, the device will start working again, for awhile, but the added pressure through the hose only opens up the leaks and holes more, untill the leaks are worse and the device again fails to get enough pressure to operate.
The 'device' in the above analogy is the combination of any or all the clutches inside the trans including the TC's lockup clutch. The spigot is the VB. The hose is the transmission with all the seals, servos, accumulators, lip seals, gaskets etc.
Now lets say that someone comes up with a new style 'device', one that only needs say 50 psi to opperate, and this is attached to the end of the leaky old hose. It will work for awhile, but soon the leaks will get worse, and soon not even 50 PSI is available to the device and it will fail too.
The new 'device' in the above analogy is a tripple lock style clutch, which in general need less pressure to lock and hold. [not a bad idea right?] But all the other devices in the trans are standard, and still need the required pressure to work. So if we add a VB with higher pressure [open the spigot] then we just make the leaks worse and soon the trans fails.
This is why just adding a TC and VB [from any provider] to a trans is not a good idea. Some transmissions [hoses] came from the factory with few leaks, and can support more pressure. But the designed in flaws and leaks are still there, and the devices still cannot work up to their potential. Even a brand new 'hose' can be a pretty bad one and have lots of leaks right from the beginning.
I hope the above helps explain what is going on in a trans. I also hope that I didn't insult anyone's intellegence. There are some assumptions and flaws in the analogy, so please don't pick it and me to pieces, I'm just trying to help explain why some things work and some don't.
Greg L. The Noise Nazi