2004:
Federer bt Agassi in the QF. Won the US Open.
1997:
Rafter bt Agassi in the 4th round. Won the US Open.
1993:
Sampras bt Agassi in the QF. Won Wimbledon.
1992:
Courier bt Agassi in the SF. Won the French Open.
1988:
Wilander bt Agassi in the QF. Won the French Open.
It has rarely happened that the player who takes Agassi out goes on to win the Grand Slam tournament. This gives some weight to your theory, Scott.