<< How do these Turbos produce boost off of idle? >>
It all depends on the size of the turbo, the engine itself, if youv'e got a turbo thats VNT style or with a conventional wastegate, how you actuate the wastegate etc. I've got 2 turbo cars in my driveway. One has a small turbo and is setup to develop 15 psi by about 2300 rpms. If there is turbo lag, I can't feel it, I touch the pedal the boost needle rockets to the pressure side of the guage. I've got another one with a larger turbo charger on it, its harder to build boost on, but I could turn it up higher for more total boost. So with it I might have more lag, but greater potential for more power once I get it spooled up. Then you've got variable nozzle turbos where the turbo acts like its changing sizes, they start out small and then the blades move and it acts like a larger turbo for a larger top end without the added lag. There are many variables that go into figuring lag, max boost, etc