try checking out setups with these subs.. I'Ve seen around 5 setups with the W7, and 8 with the IDMAX, and its quiet apparent that the IDMAX will have a higher output (at same wattage), than the W7. Both being in *ideal* condtions. Most of who are IASCA competitors.
About your final impedance // % used theory.. I wouldnt worry too much about that.. I mean, if your setup's final impedance is lets say 2ohms and your able to use your amp (max at 2 ohms) at 100%, great!
If your setyup yields a 2.66 final impedance, and your forced to get a bigger amp.. An amp that is capable of running at 2 ohms, well thats even better, IMO.. I hate pushing components to their maximum physical limits, especially amps. In this scenario, even when pushing at max , the amp will be running well below its maximum power.. So this translates to ultra clean sound even at very high intensity and very little stress on the amp..
Imagine if one day while your bumping hard, your amp kicks into thermal protection.. eww, thats scary.
Did you ever test the IDMAX?
On another note, what car are you buying?