Here is a graph showing % damage gain for DEF shred vs linear modifiers (e.g. vulnerability, dmg%, true dmg%)

Orange = Pure Fiction 4 (Lvl 85 Enemies)
Blue = Apoc Shadow 4 (Lvl 90)
Green = MOC12 / Knight (Lvl 95)
Red = King (Lvl 100)
Purple = Plight (Lvl 120)

Y = % DMG Increase
X = % DEF Shred

You will notice that DEF Shred results in a lower damage gain (in a 1:1 trade-off vacuum) for most of its curve, but ends substantially higher than linear increases at or near 100% DEF Shred. Those break-even points where DEF shred begins to surpass linear modifiers point-for-point are:

Lvl 120: 71.4% DEF Shred
Lvl 100: 83.3% DEF Shred
Lvl 95: 86.9% DEF Shred
Lvl 90: 90.9% DEF Shred
Lvl 85: 95.2% DEF Shred