I don't remember if there was heatwave, but I recall getting heatwave a lot of times, and the stamina drain seems completely normal, if heatwave really drains more stamina from foul chocobos, I can guarantee it's almost negligible. I'm at 460 max speed, 500 stamina and 460 endurance. I can sprint just fine the entire race with the passives Increased Stamina III and Head Start regardless of the weather. I'm always left with around 5% stamina left, and my average seems to be around 82 seconds in sagolii (if nothing bad happens like choco meteors, which sadly, happen very often...). I've never experienced a type of weather where I was forced to stop sprinting because my chocobo's stamina couldn't keep up, all the weathers have absolutely the same effect to my foul chocobo (or very similar at least). The only things that drain my stamina abnormally are the occasional bachus wine, caltrops and choco meteors.