Today I tested using the Core Strength mod. vs Penetrating strikes mod in a simple deul test for soley Controllers in PvP. for reference: The other player had a Toughness of 1365 (almost 20% damage reduction). i used a tap range attack from a 131 handblaster. over 50 individual hits with both mods had almost identical range of damage from each hit. both varied the majority of the time between 32 & 47. overall it seemed the core strength was just slightly weaker. this confirms the math i did before hand that suggests depending on what toughness the opposing player has, that penetrating strikes generally will come out to about 10.5/11% damage increase, but core strength is a flat 10%.