I know you've said before that you didn't use
Iridium plugs because of cost, but it might be worth it to test them out once. I put a set in my bike and got a small improvement at higher rpms. They can probably give you that little extra spark that the standard plug can't, for a few (not hundreds) dollars more.
I picked up another set of NGK CR8EIX Iridiums yesterday at Advance Auto, on sale for $6.00 each (reg.$8.99).