1. Out of the range of the AK-630: 4km or better.
2. The sensors on the missiles types are quite accurate and capable; remember, most of them (Maverick, Hellfire) were originally designed to track and destroy armoured vehicles, which are much smaller than a FAC.
Remember, during Gulf War I, the Iraqi's tried to hide their tanks by burying them into the sand, with only the turret sticking out. They did this to reduce the vulnerability to their tanks via weapons fire and to help conceal them from air attack. This didn't work; the vehicles they partially buried stood out nice and clearly on a FLIR or targeting pod (say, the old Pave Tack system or LANTIRN) from medium altitude, and they were engaged with GBU-12 LGB's. End result was a lot of blown up armoured vehicles for the Iraqi's.
Now, back to your low observability FAC's; what does this mean?
FAC's generate heat. A hot blob on the ocean below is virtually unmistakable on a FLIR. The source of the heat can come from the sun heating the skin of the boat, or from the engines and exhaust. Even then, I've seen IR video footage of sailboats at night that are occasionally used by drug smugglers; even though there is no heat source per say on a sailboat; they stood out very clearly, with a clearly defined shape that said 'that's a sailboat'. That means, I can point a weapon at your ship, as I can clearly see it.
Edit: I suggest we go for a topic split as we have clearly gone off topic.