I've never noticed the seats are too hot or too cold, nor cause any issue in hot weather, I really like Softex after over a year of use, same for my wife in her Prius. Use the search this forum box in the Interior forum, Softex has been discussed numerous times, there are a couple 8 page discussions on Softex, lots love it, lots hate it.
Tinted windows really reduce the temperature in the car so the seats are not hotter than the surface of the sun. I usually leave my car parked when it is hot and sunny outside with the sunroof in the vent position to cool the car. It really isn't as bad as I thought it would be.
Its cooler than leather in the sun, and doesn't hold heat as long as leather. Comparing it to fabric is a different discussion. Whatever shortcomings it may have compared to leather OR fabric are immediately rendered insignificant the first time you need to clean it. Unbelievably easy to clean and needs absolutely zero upkeep.