850 rwhp from N/A 347 w/ 14 to 1 compression?

I would say that yes it is *possible*. However, possible doesn't mean that it's probable for "some guy you met yesterday" to have that kind of motor. Kind of like HairyCanary said, it would take a really high revving motor and a damn good balance, and this is assuming you've got a block and internals that can take that kind of RPM and power. Maybe an A4 or Dart block or something?
 
HIGHLY unlikely. maybe people are making those numbers with built 347's with a 300-400 shot of NOS, i dont know. integra type r's have the highest compression ratio of any street cars ive ever heard of, and none of them are running 14 to 1. it seems most people with stangs dont run higher than 11 to 1. i would think a NA 347 making that kind of power would be a race only motor.
 
It's very possible. You could get that out of a 302/306 combination, but you're going to have to rev for it. A set of Blue Thunders, a stout cam, big compression, with enough manifold, and fuel would probably get you around 950 RWHP at 10,000 RPM. It could be done for about 12,000 bucks. Ask him what kind of cylinder heads he is running. That's how you'd be able to tell.

Joe