First, even though we do not specifically recommend it, there is no reason why a W7 cannot be used successfully in an IB installation as long as you consider the following points very carefully.
1) The suspension system of the W7 is not designed to limit excursion without the loading of an enclosure behind it. This contrasts with an IB specific design which is designed to tighten up before the coil crashes into metal parts. The mechanical power handling is way lower than in a sealed enclosure, particularly if you listen to a lot of low-bass intensive material.
2) The W7 may sound overly "heavy" and overdamped in an IB installation when you factor the car's transfer function and will most likely require some high-resolution equalization and/or very solid midbass support. Our IB specific designs of the past were designed to deliver a higher Q response (in the 0.7 range) whereas the W7 will be in the 0.5 range. This will bloat the in-car response below 40 Hz, leading to the "heavy" sound I was referring to.
3) If you wreck the suspension or crash a coil, we do charge for reconing.
Hope that helps,
Manville Smith
JL Audio, Inc.