That poster talked about jagged edges and some dropped frames with CaptureX, but I don't see a mention of stuttering.

In my Note at the top of the OP, I was referring to "cards" in the literal sense. You can use the regular Hauppauge HD PVR 2 as well as the GE Plus. Only the regular Gaming Edition dropped support for 5.1.

The 14Mbps recordings do show noticeable artifacts though. (As do 20Mbps Colossus captures -- there is a reason the AVerMedia hardware encoder device supports up to 60Mbps.)

All of Blackmagic's products are designed to be used in production, so they have no need for supporting compressed audio capture. Unless your STB can decompress the audio to 5.1 PCM over HDMI, their products won't help you.