when adding a power adder, i would always lean towards a slightly colder plug.
just as an added insurance against detonation, understand that the COLDER temp range is the length of the body, thus allowing a little more heat dissipation in the area around the electrode. reason i state that is the length of the plug may have ill affects. if a colder plug is not achievable, then the tuning will have to accommodate and thus leave a little power to had, but not available