For shadow bolt, 1 spell haste is better than 1 spell damage in terms of dps. It does also increase mana per second though.
However, for affliction as most of the spells are 1.5speed it isn't very effective due to global cooldown.
I used my spreadsheet,
warlock.xlsx (sorry excel 07), for calculations. Borrows heavily from other spreadsheets but might have made some errors somewhere. (21 haste rating = 1% spell haste?)
excel 03 compatiable but untested.
As for healers, it means more time in 5sr and more hps but haven't done any actual numbers on it though.