I have a 99 supercharged cobra and a 13 shelby. I think the boost occurs more under a load relationship rather than a set rpm. The vacuum/boost gauge is as much a load meter as a boost gauge. Run your car down to a slower than normal speed for 4th gear and start a progressive advance of the throttle. I am sure you will notice the gauge will go into boost very quickly. Think of it as a balancing act between how much air your engine is moving, rpm relative, and the amount of pressure the supercharger makes.
99 Cobra Red and Charcoal, KB, Tri ax, 3.73, Alum flywheel and driveshaft, JBA shorties, Magnaflow X and catback.