I'm not sure your assumptions about vapor pressure are correct. I once
saw
my blue 100LL fizzing furiously when I checked the drains on a hot
afternoon
in Albuquerque. The homebuilt owner next to me showed me his Mogas sample
and there wasn't a bubble in it.

I delayed departure until the next morning to let my 100LL cool. He flew
away without delay.

And I've flown our normally aspirated O-540-powered, mogas-fueled Cherokee
Pathfinder on days that you could fry eggs on the tarmac, at both high and
low altitudes...without difficulty.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"