Where the ground strap ends up in terms of clock position around the plug's base is a crap shoot in the plug manufacturing process. So when a plug is threaded in, the strap can end but between the mixture and the spark. This means the flame kernel that begins the burn has to "go around" the strap and that can cause a greater tendancy for low speed misfire and lost economy, lower vacuum levels, and lost power. There definately is power to be found. But, I find it a PITA, and it has to be done for every plug and each individual hole. So you need a bunch of the special washers. Change a plug, you need to re-index. Remove and retorque plugs routinely, and you may over time have to re-index it. If it's a class racer looking to back up a record, or a car with poor idle and plugs that show more than one or two are having this issue, then it might be worth doing. There is power to be found. But I dont think in most engines it will be more than 1-3%. On a 425hp street engine, that's only 10-13hp at best, and that's assuming the engine has a problem due to electrode shrouding to begin with. It's a given there's power there. But it's like removing an air cleaner... You will almost always get more power, but not a lot and the flip side (in this case) is a bit of effort.
