definitely report back. I'm thinking that if I ever change up my subs in the XLV that I'll likely do almost exactly what you're doing. a pair of dual 4ohm voice coil 12" L7's with the exile 20.1 powering it at 1 ohm.

Although, if you could comment on the idea of just doing it in a sealed enclosure for simplicity and the necessity for a little less airspace, I'd be interested in your thoughts. I've been told that I can get ~3cuft if I cut out the footrest in my XLV, so that seems pretty good for a pair of sealed L7's. I'm sure that ported they'd want more airspace, plus I'm not sure my box building skills are up to doing a ported enclosure...