Old topic but a small bit of (hopefully useful) info.
My friend has used the boost gauge output to power the consumption gauge on his turbocharged E36. He used 2nd gauge cluster PCB and used tach circuitry of it and it worked just fine after some tweaking of the variables.
I've used VDO Porsche tachometer with the boost output and it works just fine.
I'm planning to use the boost output on my 911 2.5 turbo (short-stroke turbo). The 930 tach that I'm planning to use has factory boost gauge so it would be neat and useful at the same time.
Of course, car has to be completed first.