Well the +4 plugs may not work good for your application. What fuel do you run? You are carburated or Fuel injected? You are running the MSD ignition and a hot coil, you have a 351 stroked, I dont think platinum plugs are for you.
The +4's do make a difference in the cologne v 6 engines, I can tell you that. I ran them in my 2.9l and now my OHV 4.0. I have tried autolite's, I have tried motocrafts, both regular and platinum and none of the plugs run as smooth as the +4's. Like stated above start ups both when warm and cold are improved (less cranking). I can tell teh difference and thats good enough for me....I'm on my third set and will contiue to use them. I have also ran them in a CRX and a 4.0L Jeep I6, excellent results.
Some people like them, some dont. At the track with a modded V8 they may slow you down when compared to a conventional plug, but then all you are concerned about is getting that 10 seconds......
The bosch +4 works good for one reason, because it exposes the tip of the electrode to the combustion chamber, giving the spark a direct path, doesnt seem like much but it makes a difference. Its an old trick to file down the tip of the plug, nothing really new about it. All I know is I tried them and love them, did I gain 20HP or a half second in the 1/4 mile? nope. But the truck runs smoother, has no hesitation, quicker throttle response and starts on the first bump no questions asked.....