Well, in addition to getting the 7th channel, which you say you don't need anyway, the SR-800 also has the following:
more dynamic power (front)
more power into 6ohm load
digital upsampling
39 dsp modes vs 13
7.1 multi-channel input vs 5.1
more A/V inputs
more digital inputs
choice of absolute or relative volume display
more zone 2 options
and weighs almost 9 lbs more
I don't think that 7th amp weighs 9 lbs and it doesn't come from the THX sticker so I would assume some beefier transformers and capacitors.
I'm not trying to support THX, just pointing out that there are many differences except besides the badge that everyone loves to hate
