Your calculations are good, but keep in mind this is peak power. Your devices will never ever go past this, I highly doubt an xbox ever gets close to 288watts of power usage. Something to check, however, is the RMS rating of your 300 watt inverter.. most often what is advertized on the packaging is the peak wattage, and that might put you in rough waters trying to squeeze so much from it... again, the actual usage of your devices might be low enough that it'll happily power them, only way to really find out is try them, or put each device on a bench and get an idea of the power they draw.
I'm 99% sure it will not survive crank, you could make a tank circuit to overcome this.. and the laptop should cope ok, but I urge you to look for a car adapter for the laptop.. this would take away some strain from your inverter, and would give the laptop much much cleaner power (DC to DC vs DC to AC to DC)
Hopefully this was of help.