Anyway, just for your knowledge: My plugs at 100k miles looked REALLY horrible, almost melted .... but to my surprise, I've found virtually no difference in performance or mileage once installed the new ones... so, is it really Worth it?

 

Maybe you could not tell any difference at 100K miles, but you probably would at 125K. You say the plugs were badly worn at 100K, and they would only get worse as the mileage added on.

I don't know how mountainous the area you live in is, but but driving up an incline like a steep mountain makes the engine work pretty hard and you would want everything at it's best.