Hey Geof,
My Lowden LSEII depth is just under 4in. and it has been my main workhorse, gig, outdoor/beach guitar since the late nineties. Plugged or unplugged it sounds great, excellent projection and very comfy to hold due in part to the thinner body. I also think it contributes to some of the clarity and balance. It has made me a fan of lesser depth bodies at least in larger lower bout bodies.
http://youtu.be/0PmT729448k ...this is an original quiet, slow, waltz-like, instrumental; "Dancing with Simone" played unplugged, f/s flesh and nail simply recorded on my laptop...
http://youtu.be/xhEhiDn0kCE ...then this is my cover rendition of "Turn The Page", again unplugged but with a pick and vocals that shows the versatility, projection and balance of this thin bodied Lowden. I never use effects or eq and I think the webcam mic, tho not completely doing the guitar justice, does give a decent recording in my livingroom.
Hope this helps bring a little point of veiw to your question if indeed this shallow depth Lowden qualifies in the catagory of thin-line acoustics.
noledog