Robhall,
Well, nothing bad is going to happen, but neither is something good going to happen. The Iridium tip is designed to get very hot without melting to promote the emission of electrons from the tip to the ground electrode. To be effective, there should be a supply of extra current, more than you will get from a stock coil or a coil like the 17-6822.
The dual output coil circuit is a series circuit from one plug coil connection, through the cap resistance (5K) through the spark plug (5K) through the engine head to the other plug, through that plug (5K) and the its cap (5K) and back to the coil. That's a total of 20,000 Ohms not counting the resistance of the coil itself, so once the gap is bridged, the current decreases dramatically to the point where the advantage of the Iridium plug is no longer available. To over come this you can use a coil with a higher output, like the 17-6903, which has about twice the voltage as the 17-6822 or use non resistor caps.
If you are going to use non resistor caps, then use resistor plugs if you decide to switch back to standard plugs, like the NGK BPR7ES.
Also, the dual output coil produces a positive voltage on one wire and a negative voltage on the other. Electrons come from the negative wire, so only the plug that is connected to the negative wire is really going to be more efficient.
