Critical altitude is probably sea level, yr right. You will definitely see a performance drop with a sudden disappearance of boost before you leave the ground - that and a load of black smoke!
The issue isn't only critical altitude, but keeping the combustion chamber hot enough to support combustion. When you're at altitude, low temps, low air pressure, the unboosted compression pressure isn't enough for the low CR these engines run at. Add in the SMA/Continental's air-cooling where you have nearly no thermostatic control, if you lose boost at altitude, chances are you can kiss goodbye to all combustion and you'll never get it started again until you reach the bottom.