Just a simple question to answer, no need for the debate again. I'm watching the kites in the vid Markite posted from Quebec and see the foils and the LEIs coexisting merrily so let's agree they all are good.

My curiosity is the airfoil. I know that the space under an LEI is still a low pressure area (like an extreme example of negative camber like on the Parkzone Slo-V RC planes) and I can only assume that the foils are a little bit more receptive to apparent wind because the full foil better manages the airflow generating a cleaner flow. And again, all things said, they are all good.

The question is how much airfoil aerodynamics come into play with LEIs? Just as much as foils?

Not as much - single skins never will be able to ... think regallo vs modern hang glider

Had to Google Regallo and might I say "Holy cr@p!". Hooj comes to mind! Talk about faith in science ; )

I always thought it must be really turbulent behind the LE bladder in an LEI. But it doesn't seem to be a performance choking issue. They seem to do just fine in spite of it.

nothing direct for you, but...

this is an interesting link, lol,
http://kitesurfingschool.org/kite.htm
doesn't really seem to deal with foil-lei design difference, but maybe you could drum up an answer eventually through this.

Also, maybe paragliding design could shed some light on the issue. I wonder if the foil-like design of paragliders is all practically related (weight and safety - can't have a bladder leak that high up), or if some of it regards performance related factors.

IIRC the turbulence behind the LE acts as a virtual bottom skin to a small extent ... but single-skin LEIs will never be as effective as foils. IIRC though