Your best bet is a call to Onkyo to find out if there are any differences between the one sold here and the one available in Hong Kong. They'd also be able to tell you how you'd stand warranty-wise.
But don't forget duty and VAT - they'll add up to a pretty hefty £150 or so on your receiver, and if you pretend it costs less they'll probably whip out a copy of WHFSV and check. I know, because I've seen it happen at Heathrow!
And there's also the small matter of excess baggage - the TX-SR875 weighs around 28.5kg packed, and judging from past experience with BA, Cathay and Virgin flying out of HK, the baggage limits tend to get fairly rigidly enforced.