Penetration
660 resistances equals 1% mitigation, which means 1,638 penetration is roughly a 2.48% damage increase. The Spell damage is additive to your abilities and thus doesn't have as high an impact outside ultimates which scale multiplicatively higher from your damage numbers.
On the other hand, what's typical Effective Spell Damage in PvP? If it's 8000, then 200 incremental could look a lot like 2.5%.Well, multiply that by 1.2 in most cases for the usual buff ...