Ok, I'll wade in here now...
A cooler plug or a smaller spark gap will cause a lean engine to act "richer." The reason is the burn will happen slower, especially a smaller spark gap which will act like retarded timing, and basically has the same effect. If you're running too rich at WOT, then either of these "fixes" will make your WOT problem worse and much more pronounced.
Hope this helps. BTW, what exhaust are you running? Stock backpressure will greatly simplify tuning, and cover a multitude of "tuning sins"