I have a 95 SSEi. So I should have the series 1 and you the series 2 but that shouldn't make much difference. My boost gauge also idles at -10 but as soon as any pedal is applied it starts to rise. -10 just means that the supercharger isn't forcing enough air in to equalize the internal pressure vs. outside air pressure. This is normal. My buddies GTP only shows you 0 to 10 psi so you have to stomp on it a little to register. Chances are if you bring it up to say 4000 rpm your needle should have something closer to zero continueously register.
I was reading the owners manual and the description of the boost gage suggests that the needle is supposed to operate from the zero in the middle of the gage.
The supercharger is always running and pumping in air but once it hits zero it is then forcing more air in than it could possibly get on its own. Above zero is where the power starts to come out more.
As far as the lower intake, I have blown 2 of them.
I had severe stumbleing acceleration problems as well as problems idleing smooth. Once it was replaced everything went back to normal. But if they say it still leaks and it is under warrentee, take it back and have them fix it, again.
What RPM does your car idle at and does it idle smooth?
I have also had similar troubles on non - */C cars I owned in the past that turned out to be vacum line problems. Maybe that is somewhere else to check into?