Having owned a couple of Ospreys, I noticed no difference. The connecting points of lumbar and shoulders leaves a gap for ait floe. Besides, the way I sweat, no "air flow" design on a pack makes any noyiceable difference; when tramping, I am always drenched in sweat.
Glad you enjoy the Lightning as much as I do. It's my favourite pack by far. I actually took all the little compression ribbon off mine. I doesn't need it. It's such a simple pack with an amazing harness.
I like the ribbon/webbing on the pack. And that the lose straps neatly velcro to themselves. I HATE straps flapping around everywhere! I'll say it again; greatest pack - by far! - that I have ever owned.