As it has been said the only place where heat will be an issue is where the kilt comes in close and constant contact ie at the waist. That being said I have the Black Stewart in a SWK and in a 4yrd Locharron worn both summer and winter with no problem ( I might add that even though I am in ND last summer saw temperatures up at 106F--wore kilts wool and acrylic without problems in fact I was more comfortable than tr-----d men )
As far as being too formal I wear the Black Stewart casualy without problem, in fact the Lochcarron is
a "casual" kilt made for hiking or hillwalking as they say.

Bill

(please forgive the bold type it's easier for me to read whilst typing)