I've worn my 8yd, 16oz wool tank in the Arizona desert heat with no problem. For reference we get up around 100-105 degrees (f) in the Summer, with very little humidity.

If you're going with a 4-5yd casual but still want the weight and feel of a heavier kilt, go with the 16oz...if you're looking for a lighter kilt, go with the 11 or 13oz. (not my choice, but I'm not the one paying for or wearing it).