I am not sure if I buy the whole idea of a 'spring like effect.' Now, the face buckling in a bit, I can buy, and if it goes in, it will naturally rebound outwards. But to me, knowing that the ball stays on the face for not more than .001 second, its hard for me to think that additional buckling inwards of the face would make that big a difference.
Here's a little math (which may be wrong): If the club face currently has a COR of .830 (using the USGA method where a ball is launched at a club at 100 mph, if the COR is .830, the ball rebounds at 83.0 mph) and the golf ball travels 300 yards when struck with this club, a COR increase to .84 or .85 would only result in a net gain of 3 or 6 yards. Now, let it be known, there is no physically possible way to attain a COR of 1.00, the max possible is .930, so the max possible yardage gain from COR is 30 yards.
I don't think the COR makes much of a difference really. Obviously drivers today go farther than drivers from 10 years ago, with the COR the same. Like I say, I think the spring effect is a bit of a myth. There is not a lot of 'springing' that can happen in .001 second.