Well my 2001 eldo has 47k miles on it now. And I've always been a fan of the bosch +2 plugs. So I figured I'd put a set in and report what I found.
My stock plugs were a denso brand. They looked nearly perfect. However the gaps were a bit inconsistant. Only thing I can say about thier performance was that idle seemed to be a bit rough when I first got the car at 38k miles. I'm guessing it was more likely the injectors being a bit dirty than the plugs?
Anyway.. so this weekend I put the bosch +2 plugs in. They have a sideways spark which does not shield the flame front as it propigates away from the plug tip. Every motor I've installed these in have shown the same thing. Better idle and more tip-in punch. A couple cars even had a bit better power up top.
This eldo with it's NS did'nt show much improvement. Idle is about the same, tip-in and low rpm power seems slightly better, milage is the same, and WOT is no different.
So in summary I'd say it was not worth the effort. The stock plugs were in excellent shape and SOTP performance was the same. I'm still a fan of this plug and will continue to use them. But I'd have to say that on my NS there really is nothing to be gained from them.