6psi will crack a dumpvalve off nicely dont worry
T3's come in both oil cooled and oil and water cooled. You could use the union off a GTE engine which goes to an oil cooler and run that to the turbo. This simply bolts inplace of your current oil filter mount.
In car adjustable boost just operates a bleed valve usually, bleed valves are naff and should only be used for setting up. When you have found a pressure your happy with just get an appropriate actuator. You can control how much turbo boost you use with the accelerator, if you want to save petrol don't push it down too much LOL