I used to play alot of video game..and I take note of the XBOX 360 (big supporter of XBOX products ,I own over 120games, all original, not copy)
XBOX 360 Hong Kong version is most likely to be compatible with Japan NTSC version of games.
but many Hong Kong version NTSC version games are "ALL CODES"....so you won't have trouble to run them in either the USA version NTSC XBOX 360 or the Japan verison NTSC XBOX 360...
honestly, I would buy a USA machine, because some games just never will come out Japan or Hong Kong version.
no news about whether the Hong Kong version XBOX 360 games will be cheaper than the USA version.
NOTE: current XBOX game HK version is only HK$288 comparing to HK$400 for the same game in USA verson (or US$50)