It seems to me the cars have their real power figure now when purchased and after oil change it does increase. Unlike previous GT's where their power figure was always below what it should be until oil change.
As for breaking-in, I bought a S15 Silvia, oil changed it to get 258hp, it never rose from there and now after 200km's or so it has decreased. No increase in power noted at all from breaking-in.
EDIT: Just changed the oil again in the S15 and the power is back to the original 258hp it started with, no increase overall.