Curiosity finally got the best of me and I picked up a set of E3 plugs the other day. My cost from work was only ~$5/plug, so I thought I'd see how they work in a non-stock 3800. Start-up and idle were smooth. It free-revved easily, and my AFR's looked alright.
On the test drive, however, it would misfire, cut out, and almost die anytime I put a moderate load on the car.?!?!?! I drove the car for the rest of the day like this. Last night, I pulled them out. 2 of them had a fair amount of soot/fuel-fouling, while the other 4 looked like almost-new plugs. I can only assume the 2 dirty ones were not firing properly... Oh well. At least I gave them a shot. Maybe they would be alright if all 6 fired the same, but this particular set did not leave me wanting to try them again anytime soon.
A new set of AC's, and the car is running like a champ!