as long as the car its dyno tuned, the car can handle up to 13...that's the highest i've seen so far on stock internals at least.
just to add to you question about drivability, my boosted Y8 is a blast to drive!!
i'm running stock psi (5.5) non-intercooled with the greddy evo and it spools real quick! i can't give you an exact RPM since i don't have a boost gauge, but load the turbo and you can definitely feel the torque increase.
and another thing... in 1st gear @ about 6000 RPM my tires break loose!! i can't imagine what it'll be like when i turn it up to 8 psi...