The answer to whether the jets will become a threat to the USAF
is yes. The matter for discussion is how much of a threat the jets
will pose.
The J-10 could probably pose quite a threat but that remains to be
seen. While it looks very good on paper, there have been almost no
publicised exercises with foreign jets. They haven't even exercised
with the Russians... so it's hard to gauge the actual level of
threat posed.
As for the J-12, it looks like that jet is still on the drawing
board and it is still highly unlikely that it will be operational
in the near future unless it merges with the MiG MFI program
etc...
As for whether the aircraft pose a threat to the RSAF, in my
opinion, it remains highly unlikely that the jets will pose a
threat to the RSAF mainly due to the fact that China is not
currently hostile to Singapore. Our neighbours are also unlikely to
buy Chinese jets at the moment due to the political situation
between our closest neighbours and China.
Until the 1990s, Indonesia actually considered China a mjor
security threat and had instituted defence measures in event of a
Chinese attack...
