I ended up buying a matched soundbar from the same vendor (Panasonic Viera), one that uses HDMI and connects to the ARC-enabled port on the TV, which means basically all three gizmos (HTPC+CEC running XBMC, TV and soundbar) can be controlled with just the TV remote. It also means if I turn off the TV and just use the HTPC/XBMC + soundbar for music, the xbmc mobile app can control the speaker volume.
It's all basically as straightforward and usable as it can get. If I were going to buy a Smart TV again I might get a Samsung or Vizio, as there's a Plex app available for both. Which is a whole nother topic. (I don't use Plex myself but it's what I'd recommend to pretty much anyone I didn't recommend Apple TV to, i.e. someone who doesn't have a houseful of predominantly Apple goodies already.)
I avoided the price-attractive Vizio after reading a large number of reviews of later models that suffer from persistent random reboot issues. No point in a smart TV that can't be a TV reliably IMO. The picture on my Father-in-Law's Samsung (dumb) TV is outstanding.
I went Sony only because I have a good experience with Sony products, their tech support was rated higher than everyone else's and they offer four HDMI inputs to everyone else's two. It seemed to me that I'd be bunging wires into it from all over the place and better to find I had too many sockets than too few. I'm also familiar with the Sony video family "quirks" and it seemed likely I would have a better time getting the clown out of the picture.
I think it's better to go with the direct-connected HTPC because transcode-network-decode-display is a lot more bother than just decode-display; but not having a HTPC would be attractive if your Smart TV was actually smart enough to do everything you want, and playing local media in whatever format is a major sticking point.