I find a better way to get into Zancudo is just to fly a helicopter straight into the hangar rather than parachuting in. If you can learn to land a helicopter quickly you won't get the wanted level until you exit your chopper, leaving you a good 30 seconds to get in the jet before tanks and troops show up. Once you're in it, get out of the hangar and fly straight up (don't look behind you, I've heard this causes enemy jets to spawn).
I'll have to try this out next time. I always get 4 stars as soon as I enter the military base's airspace in a chopper. Does flying low keep you off radar? I know there was a mission in single player that did that but I don't know if that works online.