Each tick is scaled by Attack Speed (sheet APS being a separate damage multiplier), making Pain Enhancer your strongest DPS gem by far in high density situations.Casting Spirit Barrage while you have 3 Phantasm s active will detonate the oldest one (with its accumulated damage).
Instead, they will accumulate that damage (as in: add up / remember what they would have done) and then release it in the final explosion at the end of the duration (with the extra damage multiplier).