I have a Force .25, I use MC59`s(Break-in), MC8`s(<=20%Nitro) or MC9`s(30% Nitro)...or if you want the best plugs a RB or Novarossi #4(Hot, Break-in) thru too about #7(30% Nitro). I have in the past used a short OS plug with the engine and it has run fairly well, should definately start on the plug!
Stick with the McCoy`s or maybe try a Novarossi for high Nitro extended WOT use
TM