Time for another spark plug post....
1) Platinum is there only for long service life. It does not help give a better spark and is not recommended by any reputable engine builder or tuner for a high performance supercharged application.
2) Iridium is expensive and isn't going to fix other existing problems. The SC has a good ignition system which will not break up or lose spark under any normal conditions. If you have ignition problems, running an expensive spark plug may or may not improve the situation but at best it's only a bandaide.
3) All spark plugs are equally susceptable to antifreeze (sudden death), fouling (not quite so sudden death), and detonation/pre-ignition (more death). Bosch platinum tipped plugs are more susceptable to the above listed faces of death than others and should be avoided at all costs.
4) For stock applications a double platinum plug will provide the longest service interval (about 60-100K miles). In this case the plugs should be replaced with Autolite APP-764's (or equivalent) gapped at no more than .050" and should be accompanied with new replacement Motorcraft plug wires.
The APP-764 is a shorter tipped version of the stock plug. It will run cooler and have less tendancy towards detonation than the stock plug. It is the OE replacement for some late model 3.8's and 4.6's such as the Mustang GT. It is the same heat range as stock but incorporates an improved tip design. It comes pre-gapped at .045" but if you would like to increase the gap slightly you can.
5) For performance applications you should use the Autolite 103 (or equivalent) non-platinum plug. This is a cooler heat range plug which will provide superior performance compared to stock and will last an easy 25k miles without gap erosion. It should be gapped no wider than .040", and .035" is typical for high boost applications (17psi and above or with nitrous). This plug is available in double platinum (APP-103) but again, platinum is not recommended for high performance applications.
The Autolite 103 is an OE replacement plug for the 2003/4 Cobra, all Lightning applications and the new GT. Obviously Ford knows a little something about supercharged motors and it should be noted that the SC is the only supercharged application for which Ford used an extended tip plug. It is my understanding that this was done to reduce idle emissions. It certainly wasn't done for performance reasons.
6) For any car that has been modified and hasn't been tuned or is in the process of any engine buildup or modifications an inexpensive standard copper plug should always be used. If you damage a plug for any reason (such as fouling, for example) your $12 Iridium plugs will become garbage just as quick as a $2 Autolite. Do yourself a favor and use cheap plugs at least until you have eliminated all other possible problems. Additionally, anyone who thinks they have a performance SC MUST get used to the idea that they will be changing or at least removing their plugs at least once a year or 20K miles. If you are not removing your plugs to see how things are going in side the cylinders you are driving with blinders on and just asking for trouble. A performance vehicle should have the plugs removed regularly to check for abnormal combustion. This is an excellent time to simply install new plugs. A brand new Autolite 103 will perform better than any 3 yr old plug. Why do you suppose top racers replace plugs after every race?
7) This is also a good time to talk about plug wires. The OE ignition system was designed for a carbon core, high resistance plug wire. The DIS multi-coil system has plenty of "juice" to fire the stock wires and in fact, some engineers will argue that the OE plug wire resistance is an integral part of the ignition system and reducing wire resistance may have a negative effect on the performance of the system. In any case it has not been shown that any low resistance wire is better than the stock wires in any case. The mindset of "high performance plug wires" comes from a long line of distributor fired engines that relied on one coil to fire 4 cylinders per revolution. Coil saturation and dwell time as well as total coil output were a major concern. In our case with three coils, each firing once per revolution, we do not have an issue with saturation and the coils have plenty of dwell time at any rpm our engines are capable of.
While spark plugs certainly can cause an ignition miss, most often the problem is installer related (cracked procelin, improper gap, incorrect tightness, or contaminants on the electrode) and not related to the plug itself. An exception to this rule is the Bosch platinum plug with the thin platinum electrode. These plugs have been proven to give poor performance on any and all supercharged applications.
Plug wires, on the other hand, cause the majority of all ignition related problems. No plug wire brand is immune from problems and consequently over time it has become my recommendation to use Motorcraft or Taylor wires. They may not be any better than others but they are certainly cheaper so you won't feel so bad if you damage one. In any case the wires should clip on with certainty. If you cannot feel for sure when the metal clip engages the spark plug then you must try again until you feel it. Muscle and force will not make them work better, instead it will damage the ends and you'll end up buying new ones. Never say to yourself "I think I got them on right". Either you do, or you don't, there is no "maybe" unless you don't mind doing the job several times.
Also, always use the silicone grease provided with the plugs. Not only will the wires snap on easier but it will be possible to remove them later if needed without tearing the boots.
Other components of the SC ignition system can cause problems as well so don't write them off as sources of your problem. DIS modules are common problems as are cam sensors. Bad grounds (damaged wiring) is also a source of problems but the first and most common place to look is plugs and wires.