I have heard the P7 and it is very nice indeed. The hidden beauty of it is that you can biamp speakers by using unused channels (eg: in a 5.1 setup).
The market has needed a high quality 7 channel power amp in one box since the arrival of THX EX. I'm surprised only Arcam and Lexicon have made real inroads into it in the UK.
The NAD amplifier you refer to is for US type custom install projects (multiroom audio) rather than for one-room AV use. At 75WPC it is more powerful than the usual offerings here however.
So far, thank goodness, 7 channels looks about all we all have to cope with for home cinema systems.