First of all, you can't assume that "the amps watts double as the impedance halves." Some do, most don't. Second, there's no standard technical meaning for "recommended power," so every manufacturer probably means somethng different. (If I were a speaker manufacturer, I'd assume that most consumers only look at 8-ohm power ratings, so I'd use 8-ohm power ratings for my power recommendation, even if they were 4-ohm speakers.)
I'd e-mail Thiel and ask them what they recommend in terms of amp power for that model, and whether they mean into 8 ohms or 4 ohms.
I'd e-mail Thiel and ask them what they recommend in terms of amp power for that model, and whether they mean into 8 ohms or 4 ohms.