I don't know what the safe working amperage for the aux switch circuits are other than what Ford has them Fused for. I wouldn't go any higher because electrical components get expensive. What I would do instead is have the aux switch control a heavy duty relay with the proper size wiring and circuit protector. This way you can design the circuit for as many amps as you want 30, 40 50, whatever. Just be careful you don't over work the alternator if you get crazy. What mods did you have in mind?
and there's your answer.....