I used an MSD coil, not the Screamin Demon. Like I said before there is a noticeable difference in voltage between the MSD and stock coil (but it's not like thousands of volts). You can also "visually" see a difference in the quality and strength of spark by grounding the plug against the block and firing it. Perhaps my stock coil is out of spec (I didn't test two different stock coils) and I don't have a shop manual to verify the range. What I do know is the car runs fine with the stock stuff, but once you modify the electrode (which also facilitates a wider gap and a radically different ground electrode) the engine runs erratically, misses and idle quality is no good. With the MSD coil and wires it runs just fine. The actual power gain using the mod and indexing the plugs is approximately 2-3% which is about what I expected since I've done this mod to alot of different cars. Typically you see 1-2hp per cylinder depending on combustion chamber design, compression and intake/exhaust valve/port efficiency. It's not alot but it's still something... and on these cars every little bit helps!
I can see this working better than using an open style spark plug like Brisk for instance because even though it is "open" it still uses 4 points for the negative electrode, which means if the path of least resistance is the negative electrode on the back side of the plug, the actual extended length electrode of the longer plug becomes a restriction. With a standard plug with a single point electrode, you can actually point the plug directly at the exhaust port (the whole point in indexing plugs in the first place).
Yes, I have plenty of dyno runs of before and after and then back to stock again (12 pulls in all). The only mod I had at the time was a K&N panel filter in the OEM SVT airbox. I saw 148hp and 137 pft at the front wheels (average of first four runs). After swapping out the parts (which took 15 minutes), the average went to 152hp & 141pft (four runs total). Back to stock again and 20 minutes later we did 4 more runs and the average was 147hp & 135 pft. Pretty consistant I'd have to say. This power increase was pretty consistant from around 2500 to red line so yes it did add some power under the curve (and 4-5hp peak)
I plan on doing dyno tests every time I make a change. I don't have access to a scanner at the moment so you'll just have to take my word for it for now. However when I'm completely finished with this project (maybe spring/summer of 2008) I will make all my documentation available for everyone to read (including the results of removing as much weight as possible).