From my parti gyle experience i would say your loss would closer to 25% or more.
My first runnings totalled roughly 6gal with an OG of 1.091.
I filled the mash tun with water on the same grain and it sat for an hour while i boiled the first runnings.
The second runnings from the mash tun had an OG of 1.040 if I remember correctly.
So, i would definitely sparge. Either that or parti gyle.
If you haven't tried parti gyle brewing, you should try it. It's very interesting just how different both beers taste.