I think it's reasonable to downstep to the 5D III. I'm always amazed at the clean creamy smooth quality of OOF backgrounds with the 1D X, but the 5D III's backgrounds are still very good, and it's subject detail is better. I think there is a lot of value to having more pixels, especially with full frame, since it's tougher to get the needed reach sometimes. Cropping is just a necessity.
I also fully understand the negatives of a loud shutter. Just the other day, I was photographing a Black-crowned Night Heron, and the shutter sound of my 7D (which is also relatively loud) spooked some deer that had crept up between shots...I hadn't even known they were there before, and if it wasn't for the shutter sound, I doubt they would have cared that I was there either (I probably looked like some oddly shaped tree with all my camo). After the shutter sound, the deer refused to cross this little rise I was standing on to get to the water...and I love photographing the deer drinking and splashing about
Personally, I use evaluative metering in manual mode and a fixed white balance setting (I don't like my WB changing shot to shot with minor changes in the light), and I manually compensate, so not having AF-point linked metering or AWB wouldn't be a big deal for me. I use Art Morris' approach to exposure, which is actually pretty awesome. A lot of the time, so long as the lighting and angle of light doesn't change, once you set exposure, you can just keep shooting until something does change (i.e. cloud, angle changes too much, etc.)
The only real serious drawback of the 5D III vs. the 1D X is having half the frame rate. I consider 6fps rock bottom for action shooting, especially for birds. I would much prefer to have 8fps, and 12fps would be perfect (not too many frames, but not too little either.) Assuming you frame the same, there really isn't that much of a difference in real sensitivity (the 1D X has 44% Q.E. while the 5D III has 49%...the smaller pixels means more readout wiring in the 5D III, which reduces total light sensitive area a bit, so in identically framed situations, there really isn't a significant sensitivity benefit for the 1D X, and better cropping ability.)
I've considered what full frame camera to buy on many occasions. I'm using a 7D now, and I like the small pixels/high spatial resolution. I'd like 12fps, but seeing is that is the one significant benefit to the 1D X for me (there are certainly other benefits, but none as significant as that relative to the 5D III), I've decided to get the 5D III. The savings will allow me to invest in my astrophotography (probably a cooled mono CCD camera, which will suck up most of the extra funds I'd save by not getting the 1D X), and the 5D III is an excellent general purpose body (good enough for birds/wildlife and has the best Canon AF system, better for landscapes than pretty much any other Canon camera (large frame and the most pixels of any Canon camera), larger frame so better for low light shooting, will work quite well as an astro cam
until I buy the mono CCD, etc.)