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I'm not sure I understand where you got your APC, Phil... you mention both SPG9 and Swedish Dynamics. I have the SPG9 APC in my car, and I can tell you that it performs boost reduction to protect the engine when sensing knock just as well as the stock unit. When I first installed it, and prior to any other mods I did to my car, it allowed only about 1/4 redzone boost for me, with knock sensing kicking it around just above the red/yellow line under sustained boost. It wasn't until after I did fuel and ignition mods that I could sustain higher boost pressures. Once I eliminated the causes of knock (lean mixtures and not enough ignition retard under boost) the new APC would allow excursions to the top of the redzone.
Higher pressures can cause things like head gaskets failing sooner or (as I've already found) hoses popping off the intake manifold under higher than usual boost. As Jordan says, the extra torque is going to be "felt" by the clutch, tranny, and drivetrain, thus there is potential for those items to wear or fail sooner than normal, too. They weren't really designed to handle the higher horsepowers the engine is capable of producing.
So far, I've suffered no ill effects from my own APC mod. If your engine is in good shape to begin with, I don't believe the extra boost pressures will cause much harm. The big risk is pre-detonation, or knock, and you're still protected from that.
How you drive determines how long your car lives. If you flog it and use wide-open throttle at every traffic light, you'll be making repairs sooner, rather than later. I regard the extra boost as an emergency reserve, and rarely use it, just as I rarely ever need to use maximum effort braking. Get used to watching the boost gauge and the tach as you drive and shift -- with practice, you can keep it out of the yellow in day-to-day driving, only engaging the turbo when you really want it. If you show a little restraint when you're driving, a modified APC shouldn't cause any significant harm. Many of the people who read and post on this board have done APC tweaks -- we'd be warning each other NOT to do them if they were very bad for our cars.
You might want to have another mechanic, one familiar with Saab turbos and APC tuning, give you a second opinion.
- = M = -
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