how does an adjustable brake proportioning valve INCREASE rear brake pressure?
it works exactly the opposite of that- it is like your fuel pressure regulator. turn the knob out, and it allows the full brake pressure that the master cylinder is putting out to get to the rear brakes. turn it in, and it decreses the amount of pressure that gets to the rear brakes. but, just like your fuel regulator, it can only put out as much pressure as is going into it. it isn't a pressure multiplier.
it can be installed after the stock prop valve- but it will only help if your rear brakes are locking up under hard braking. it allows you to turn it down a bit so all 4 wheels are stopping the car instead of the rear just sliding along.
i had one in my Nova, and it really helped a lot in keeping the stock rear drum brakes from locking up when the C4 Vette front brakes grabbed and shifted the weight forward and unloaded the rear tires. with it all the way open, the rear brakes would lock up almost instantly and the car would get all slideways. after fiddling with it a bit, the car would just squat down and STOP. and this was without a manual configuration without a brake booster of any kind.
so, to summarize, and adjust able prop vavle will do NOTHING to help a turbo Buick hold boost a the line, but it will help it stop safe and straight at the end of the track once it is properly adjusted.