I saw two concerts on that tour - Brighton and one of the London gigs. A few people booed Horn at the former, and at the latter (Rainbow) one 'wit' yelled 'bring back Wakeman' before the Downes solo. Geoff gave a one-fingered salute in return... A couple of beer cans were thrown onto the stage, too. So there was some barracking, but nothing like as much as has been suggested... Mind you, the other UK shows could have been different.
(There were also reports from Brighton of people storming out during AYAI and demanding their money back. I can't confirm - though I certainly saw quite a few people leaving throughout the evening...).
It should be added that quite a few people I chatted to casually before the concerts didn't know that Anderson and Wakeman had left....
On the basis of my first hand experience, plus tapes of the US Drama tour, I'd have to say that Horn's voice had deteriorated very significantly by the time he reached us. It was pretty embarrassing, to say the least, though Squire did a heroic job in trying to cover for him You had to feel sorry for Horn.... but I can well appreciate why a number of people felt cheated. Not that this excuses the behaviour of the inevitable moronic few, of course.
The publicity here certainly wasn't too explicit. A couple of people remarked to me at the concerts that "since the publicity used a Dean picture, it never occurred to me that this wasn't the old Yes..."