people always forget, when citing previous consoles and prequals, that the first "new" game on a "new" gen always sells higher than normal. For example. GT Prologue compared to its PS2 counterpart. DMC compared to PS2 counterparts. There is an aura of excitement, newness, freshness and intrigue that pushes sequels debuting on new systems higher first week. If the game happens to be good, as MGS seems to be, it will most likely stabilize higher than expected.
Id say 400k first week roughly 1 million LTD.