[Shane@DBPerformance]

#1 Different for every car. Depends on the gas you run, how well your intercooler works, how easily your motor likes to knock, etc. How you tune the timing and fuel will greatly effect what you can get away with for boost.
#2 Who knows. Uncrushed, maybe 17psi. Crushed who knows, some get worse with crushing some will hold low 20s with crushing.
#3 Maybe 28 on race gas.
#4 You shouldn't boost it at all, if you want good gas mileage. The more boost you run the more fuel you will need. Try not to boost at all in normal driving. Put your boost controller at whatever and don't give it enough throttle to make boost and waste gas everytime you try to accelerate.