If I find that I'm having spark-blow-out under high boost, then I'll look at ignition upgrades. For the SRT-4 I used MSD wires and an MSD coil.
You should be ok. I doubt that you'll have blowout at .28.
Something to keep in mind with that gap on a iridium plug. The manufacturers used iridium because it is hard. That translates to a long lived spark plug. The iridium plug has a VERY small electrode tip and that combined with the VERY HIGH RESISTANCE of the iridium itself, you're going to end up with a reliable, but very small spark kernel.
The makers of this chart made the comment that they left the iridium off of the chart because of its extremely high resistance. They postulated that the iridium should have been left in the ground.
Electricity travels around the outside of a wire. so a normal size electrode in an iridium plug would encounter very high resistance trying to get to the tip of the electrode.
The only way for the plug manufacturers to allow the iridium plugs to fire was to make the tips very small. That makes the outter shell area of the plugs center electrode smaller, so the current encounters less resistance on the way to the electrode tip.
So, as I said, the result is a reliable but smaller flame at the tip of the lug.
Silver is obviously the best metal to use to make spark plugs.
Jerry