I've been thinking about this quite a bit over the last couple of days.
My roommate has similarly designed Focal tweets and he's religious about his mounting in the sail panels. I'm quite happy with the soundstage produced with the stock locations (given proper aiming), which seems to sit right about the center of my dash (with time correction).
When I sit in his car, the sail panels seem to produce a much brighter sound, even with his -6db of attenuation on them. I'm not a big fan from that little bit of anecdotal evidence. I may experiment with placement of my own tweeters, but I'm more than a little skeptical.
Any of you in the Bedford/Waltham area willing to demo the joys of your sail panel install?