Definitely a spike is not a good idea. My own thinking is that corals, like fish, acclimate to varying readings in the 'safe' zone, but very, very slowly. If your alk has always been 8.3 and it gets to 12 over the course of two weeks, you're in danger. If you rise or fall a few tenths of points, no great problem.
I also suspect that there is a general 'safe' zone between 8. something and 9, and that if you get above 12, things can go bad with just a .1 increase. I can't prove this, because I have only my own tank to go on, but I know that my tank was bulletproof so long as I was around 8.3, and when I got above that, and got to 12---yes, fairly fast, over a 2 week period---disaster was right around the corner. I think I could have gone to 9 or 9.5 safely and then back down fairly fast; but more it went to 12--- maybe too many points of rise, or just that the higher it gets the worse the reaction.