i have no idea what you're idea of SD is, but SD gives you more virtual DP... it doesn't mean you take less damage, it just means you can take more of it.

i don't know when exactly the merge happened, but i got frustrated with the new version, because bitdefender really hates the new game... have to disable it before i can play.. 

Now then.
Even with your 900 SD, you would still take 2500 damage from a grenade that does that kind of damage (+ criticals...)

The effect of SD is to take that damage not directly from your green health bar, but mostly from the blue arrow (on your ingame health bar) that goes down, resulting in burning damage. Your 900 SD actually translates into 10% damage taken directly, and the other 90% from the blue arrow that will be repaired until the level of the red arrow.