I own a Casual Kilt from USAK which is Marton Mills 12oz PV (just shy of 5 yards). It does get warm in 100* F weather (but so are shorts).

I also have a 16oz Wool kilt (5 yards).

Both are knife-pleated.

Weight of the fabric, the properties of the material, pleating style, and the yardage used seem to be the deciding factors.

Some weather is entirely too hot for a kilt...any kilt. It gets over 100* F here during the height of summer.
Some weather is too cold for a kilt as well. Last night it was 29* F in my neck of the woods. Tonight it may give us a light dusting of snow (fingers crossed).

Just my tuppence.

As for USA Kilts quality, customer service, and prices you will be hard-pressed to find a better kilt for the money here in the US. I wholly endorse Rocky and his products. You won't find a better kilt for the money unless you go fully-bespoke (which I'm sure Rocky will eventually add to his inventory, seeing as he's started offering box-pleated kilts recently).