Manu has had great moments in the clutch, and he's been phenomenal at times. I think it's really attributable to his heart and will to win. The guy just hates to lose. Unfortunately, that has also led to mistakes in the clutch like the foul on Dirk in 2006. But, again, it's because he's willing to leave it all on the court in those moments.
However, I do think the dynamics of the team really do help his clutch statistics. How many teams are going to leave Duncan and Parker to double team Manu? And, of course, the Spurs have traditionally had good three point shooting at the other two positions with Horry and Bowen and Finley and Barry, so double teaming Manu and collapsing the defense becomes almost near impossible for any smart team. In those situations, Manu often has strictly one-on-one situations with maybe a little help defense that generally comes late when he attacks the basket. In contrast, guys like Kobe, Wade, LeBron and others will get double teamed and triple teamed well over 90% of the time in those situations.
It's still a great tribute to Manu that he can still perform well and shoot the ball well in those situations. I'm just saying there are helpful factors because of the team he's on.