We've been across and back a few times and never get the cabins we want. We pay for a deluxe, get downgraded. We pay for a double, get downgraded to a quad. Staff don't seem to be at all concerned, and there are no refunds. I actually wonder if the "upper" level of accommodation even exists.
If you don't want a plateful of warm, mass-produced food, have one of you hop out of the car as soon as you get on board and make a dash up to the restaurant to secure a booking.
And definitely sign up for the mobile alerts to avoid wasting hours waiting in a car-queue to board. The Spirit of Tasmania RARELY leaves on time. (Date of experience: Feb, 2010 )