If the computer is left without power for.

Doubtful. The short and long-term fuel trim numbers will be lost from memory so the fuel / air mixture won't be right at first. Those "look-up tables" will be rebuilt as soon as you start to drive it and you'll never notice that. The biggest thing is the computer will have to relearn "minimum throttle" before it will know when it must be in control of idle speed. Until that takes place you may not get the normal idle flare-up to 1500 rpm at start-up, and the engine will likely stall at stop signs and be hard to start unless you hold the gas pedal down 1/8". To meet the conditions for the relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the pedals.