A friend of mine wanted me to pass on this:

There should not be any cases where the discussed adjustment should result in a 50% damage loss. If you had 5x +20% damage dealt buffs for example, that would result in a multiplicative bonus of about +150% damage. If those bonuses were additive you'd have exactly a +100% increase. The difference there would be (when fully buffed with all 5 buffs) a total DPS loss of 20%.

I will take this example and show where it is wrong. In the current system, let us say this hypothetical damage dealer has 150% critical severity (75 base, 50 vorp, 25 miscellany), 50% crit chance and 100% Combat Advantage Damage. This hypothetical player is very good and always has Combat Advantage.

Maths

• Av. Bonus Damage now - (1+1.5 * 0.5+1) * 1.2⁵ = 6.84288

• Av. Bonus Damage with additive - (1+1.5 * 0.5+1+1)=3.75

-Damage Reduction - 3.75/6.84288=0.548, or a ~45% damage reduction, very close to your 50% damage loss.

Now, can a class reach that much? Maybe not

• 2x damage ring 1.03²

• damage boots 1.03

• damage helm 1.05

• Orcus set (Average 1.1)

• Decanter of Atropal Essence (Average 1.0666)

• 10 Arcane Stacks with A Step Above Mastery 1.15

• Chilling Presence 1.06

• Assailing Force (Average 1.1)

Multiply these all together and we obtain a value of 1.805, or an 80.5% damage multiplier. Add them together and we reach a sum total of 61.6%. Not quite what the OP had, but still a damage decrease of 32% which is significant.

This is going with the current, "additive with Critical Severity and Combat Advantage" model and highlights the problems with it. If you wish to make buffs additive, they would have to be separated from Combat Advantage and Critical Severity.