Well, I really can't answer your question, other than to say the GS's that I played all had a lot of volume. I've never played anywhere but my living room (and the guitar store), but if a GS doesn't have the volume for what you need, then probably nothing will. I almost bought that GS6 it sounded so good, but the body was just too big for comfortable couch playing. I play a Martin a couple of Martins and have a GC3.
Just to clarify (I reread this thread and I'm still not 100% sure)... The one you are looking at is a GS size body (16.5" lower bout) with a slot head, right? For Taylor, GS=16.25" lower bout, GA=16" lower bout, GC=15" lower bout. Your first post makes it sound like a GC, but maybe by "concert size body" you just meant the shape (?).
Rather than me rambling on about what I don't know, I'm sure the guys at the Acoustic Guitar Forum (formerly Taylor Guitar Forum) could give you a definite answer about the unplugged volume:
http://69.41.173.82/forums/
Oops, sorry I just checked that link and got this message...
5-24-08 The Acoustic Guitar Forum suffered a serious hardware failure this morning. At this time, our fine web hosting provider is working to get our community back up and running as soon as possible. Thank you for your patience.
Looksl like you're stuck with UU for now.