I put them on this fall, noticed a slight difference (I think).
If it wasn't such a pain to change plugs, I'd try it on the mountain and run them back to back, but I'd rather ride than wrench.....
I'll take those guys word for it, they spend all summer trying to tune/tweak to gain an edge for the grass/asphalt. If they say it gains a few hp mid range, I believe them. Last time I checked Mike Knapp isn't making any money selling spark plugs, so I don't think he has anything to gain by trying to get everyone to buy a set.
tk