First of all: There is absolutely no gain to use IW over Invocation if you use IW actively..
You're implying that each absorb of Incanter's Ward is proccing a full 15% damage for 25 secs as well as your IW proccing every 25 seconds. That is not necessarily the case in raids. Like Methusula said, for the entire pull (when all your procs are up) you're losing that 9% damage, which is inflated by int and haste procs until you take enough damage for IW to proc the 15%.
With the levels of haste we are encountering (talking Invo cast times here), especially a haste-stacking frost build, the DPS loss for taking IW over Invo will be much greater.