As far as heat tolerance goes, I suspect the temperture of the soil is a big factor to consider. Your soil is probably relatively cool compared to areas with long hot summers.

I have also been observing the heat tolerance of my passiflora's. Mainly with my tacsonias. X exoniensis, manicata 'Linda escobar', and 'Susan Brigham'. They seem to grow fine if dappled shade to bright shade. They get morning sun for a few hours and then an hour of light just before dark. When that 1 hour of late afternoon sun hits them the top 6 inches of growth nods down and wilts but once the sun is off of it they perk back up. They are still growing very fast but did drop the flower buds. We had a few days of 105 F and the buds dropped instantly. Hopefully they will resume blooming in the fall.

passi4thewin please keep me updated.! And your hunch ( and mine, too, I suppose) is correct: the membranacea is in almost full shade and the tarminiana gets full sun for 7 hours, and then partly shade. We have excellent air circulation and I live dead on the 32nd parallel, with about 1/2 mile of uplands separtating my house from the marsh to Wassaw Sound and the Atlantic. Soil temperature is currently 81.