Great thread, gang; thanks for the excellent info, Bruce.

I recently installed a GTX-335 in the Extra and see first-hand evidence of position drop-outs whenever its GPS antenna loses its view of the sky. (The transponder reports loss of position data via an explicit error message.) In line with Doug's experience, it takes about 5 minutes for it to clear and return to normal operation. Flightradar24.com – granted, not a definitive reference – seems to confirm this via gaps in the lateral track.

If I receive an NPE letter from FAA, I'll post my reply here as a template for anyone who might want to cob from it.