I like running #7 plugs unless ambient temps go much below 60F. In high humidity I don't think a #8 would be all that beneficial to control timing and have the engine maintain it's performance. I like to run the #7 plug and just take 0.1mm shim out in real high humidity.

Low altitude AND high humidity/temp (FL) is real fun for a guy who is from NY and normally races in Canada. The altitude at sea level is the predominating factor in my decision making. More air= adding a shim to prevent detonation and keeping the #7 plug to keep the flame lit on the low end with the humidity. There are really three facors to consider in this condition...... altitude= add a shim, humidity= remove a shim, temp= lean the mixture. The best thing I have come up with during a lot of hot laps is to add a shim while keeping the #7 plug. My needles are usually with 1/4 turn based on temps.

ask some guys that travel up north to run what they do..... probably the opposite of what I do as far as shimming.