Splitting IAD off from SIGINT wouldn't reduce the number of zero-days the government collected, but it would:
* Ensure that the advice IAD was generating was untainted by SIGINT influence
* Enable IAD to independently collect vulnerability intelligence and disseminate it (most importantly, to vendors) without having to endure a bogus equities process to ensure they weren't blowing a SIGINT operation.
Of course, this only works if IAD is stripped completely out of the NSA, and perhaps out of the DoD entirely. IAD probably belongs under DHS.