Crit/Damage Increasing Modifiers:
Shadow Word: Pain snapshots whatever your Crit % was, as well as any damage-increasing modifiers that were on you (Shadow-weaving, shadowform, tricks of the trade, etc) when you cast it. These stats are preserved through refreshing it with MF.
Spellpower:
SW:P is supposed to (but does not on Warmane) snapshot your Spellpower when it is cast. However, this snapshotting is supposed to be unique in that when you refresh Shadow word: Pain with MF, its damage should be recalculated with the new spellpower that was present on you when you refreshed it. On Warmane none of this spellpower snapshotting works as described, and each tick simply ticks dynamically based on whatever your spellpower is at the moment.
Haste:
SW:P does not benefit from Haste at all :P.