By the way-this July 4 1980 performance was "fixed" prior to airing on TV. (as was Knebworth). I have heard the unsweetened actual live version and the performances are a lot rougher!!
Very true; the radio broadcast of that DC show is more raw and rough. I've probably, partly out of sentimentality/nostalgia, watched and listened to that show (both the original radio broadcast and the completed TV special) more than most any other, and I can say that, to my ear, the actual overdubs for the TV special were not that excessive.
The main overdubs I hear is an extra electric piano/keyboard, and some sweetened backing vocals in spots (the intro to "School Days" most obviously). But all of the lead vocals and main instrumentation are untouched. I think the main thing they did in post-production was to mix in to excise bad bits, and I would imagine the unsalvageable nature of certain performances (Brian's batteries running out mid-vocal on "Wouldn't It Be Nice", Al's momentary frog in his throat during "Lady Lynda", etc.) were what dictated which songs made the hour-long cut.
I wore out my original 1980 videotape of the HBO broadcast for eons, and then got a French Beach Boys fan to buy a Japanese DVD import at the Virgin Megastore (in Paris I believe), and it still blows my mind how good it looks, being that the show was shot on just normal SD videotape that is now 36 years old.
More trivia: I recall Alan Boyd telling me years and years ago that the raw footage from the concert was missing/lost, but I'm thinking in recent years they did find the raw footage, as a quick video-only clip of "California Girls" (not aired on TV) was used in a C50 TV ad.