Well, you might consider the actual history of the two clans? You didn't say what your "true" clan is, so it might be prudent to take into account their history, if any, with the Sutherland Clan. Personally, given that you are going to the games to affiliate with YOUR clan and that a clan's tartan is the utter outward expression of affiliation and not to mention the still sometimes touchy rivalry of Highland clans, I would wear my affiliated clan's tartan- that is most appropriate. Even without any "history" to speak of between the two clans in question, folks are apt to be "put off" if they become aware of your anomaly. You might want to be prepared to do a lot of explaining, which I'm guessing could be an uphill battle.

Furtherto, I wouldn't honestly think that there would be a great difference in how hot your kilts will be if they are both wool (it can't be THAT hot in Pennsylvania, can it?). You could compensate by modifying your footwear and what you wear above the kilt to help stay cool, to wit: don't wear a belt, wear short sleeve shirt or t-shirt, don't wear hose (ie sandals or flip-flps) or scrunch down your hose or wear hose with considerable less wool content. And drink LOTS of water. These are all things I consider given warm weather, and here in South Georgia (CSA), it's a BIG consideration...

Happy gaming!

Cheers,
Wesley