All you're doing then is fiddling with probabilities. You can do that just as well by changing the length of the sha unless you have a reason to think sha is broken.

However, the probability of a hash collision is already incredibly low. As a demonstration of this, here's a list of a tiny subset of bitcoin private keys: http://directory.io/

The human mind just isn't capable of understanding how large of a number the number of sha hashes is.