If fuel usage is way high, definitely take it back and tell them. It may need a program update or something. Just be sure it's really using more fuel than usual, enough to be significant.
My boat was using too much fuel, but it was due to a bad thermostat that wasn't letting the engine warm up enough.
What warning light are you talking about? The V-drive light is supposed to come on in neutral and low rpm's. If it's an engine warning light, again I'd have them fix it. If it's a wiring fault, they should fix it. You shouldn't have to live with the problem.
The Illusion-x comes ready for lights, but I'm not sure of the tap size, sorry!