It depends how long the temp spike is before it's caught and corrected. A high temp spike that stays up long enough to drive the inside of the egg temp up past the danger zone will and usually does kill the developing embryo. A high temp spike that is caught before the internal temp goes too high isn't as detrimental.
Yes, eggs that are incubated at a slightly higher temp over the average of incubation will hatch out early and eggs incubated in a lower than average temp for the incubation will be delayed. However, In both cases you have higher risks of chicks being born with development issues and strength issues as well as possible hatch complications.
