just my 2cents, but i'd go with a 2.35 format screen with the jvc(though not necessarily saying you should choose the jvc, just that i don't know much about the panny).
with the room dimensions you have, the 16:9 screen wouldn't be much(any?) bigger the 16:9 image you project onto a 2.35:1 screen. so you get no advantage for 16:9 content, and end up with smaller than you want scope movies. the jvc handles CIH pretty well imo, there may be a bit of drift over time, but it's easy to realign, and it's black enough that any dark colored paint on the screen wall will not show the overspill.
as for where you mount the projector, i've heard that there is no compromise. that is, mounting it in the middle only gets you the worst of both worlds. the benefit of improved contrast at the long end tapers down to 'bad' contrast by the time you hit the middle, and the benefit of a brighter image on the short end tapers off to 'dim' by the middle as well. so you're best to decide what you value more(i went for contrast and i found the jvc to be exceptionally bright) and try to get it as close to that end of the range as you can. just be careful, with the jvc it needs to have room to zoom to switch between 16:9 and 2.35:1 if you go CIH. so if you set up for a short throw, use the 2.35:1 setting, and if you go long throw, use 16:9 setting
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1130K, 7.1/5.1.2 audio
Sources: HTPC, PS3, XBOX360, Wii
Control: Harmony One