it only makers a difference in hot weather...under normal conditions, in the morning or cooler weather you'll notice that your truck is more peppier...when it gets heat soaked or if the weather is hot the truck will feel a little sluggish. So the powerwire does trick the pcm into thinkin it's cooler and adds a little fuel.. I ran a powerwire years ago and it did help in the summer months, winter months you can't really tell so i removed it during that period. Youl really need some way to datalog the a/f's to tell if it's performing better. I liked using it because it gave me peace of mind as I was modding up (I'd rather run rich then lean). I basically ran it til i was able to custom tune on my own and had no need for it.
Also is only valid for closed loop. At WOT the computer reverts to open loop and runs without input from the sensors. Wouldn't help at all on WOT even if it really were beneficial.