Invul Pierce is either a yes or no from the get-go. There's no real benefit of having it for longer or shorter because the series of questions is:
Does the enemy have an Invul from Break effect? Y/N
If N, does the enemy have Invul as part of their kit? Y/N
If N, it's totally useless.
Like, I get that Invul Pierce is strong when you need it against an enemy with Invul. But that's the catch and also the problem of the effect, you need the enemy to have Invul for Invul Pierce to do something. Meanwhile, the current welfare CE with 50% charge and 5% less Power Up is just better as a more general use CE for Servants that matter.