so the effectiveness of the bag would only be noticed without the sleeping bag?
in that theory then wouldnt it be best to get in it starkers? 
Even tight around a sleeping bag, the bivvy bag still operates as an windproof bag, sealing in air, reducing convection etc. That is worth something, potentially quite noticeable. But the value of its reflective properties would be very much reduced. If it were loose around the sleeping bag and crinkled then the air gaps would allow reflection.
Starkers would give you the best reflective effect as your skin temp is the hottest surface in comparison to the surface of thermals. Hotter surfaces radiate more. However I bet it still wouldn't be as warm as wearing clothing inside the bag. But the effect in terms of being noticeable would be potentially quite dramatic.
i would use inside a tent without the inner so i would want a bivvy anyway
i have seen couple of reviews where a guy said he was fine in just his t-shirt at 29.F
durability may be an issue with it but it does have a zip so getting in and out shouldnt put much pressure on the seams
as for the loft concern wouldnt this be a problem that concerns all bivvy campers? ive used a lightweight (350g) bivvy that was snug and im not sure if it affected the temp but getting in and out was a pain as the sleeping bag would have to come partially out of the bivvy to get in and out and during the night would end up creeping out of the bivvy too
I've seen a review where the guy was too cold to continue in the low 30's wrapped up warm. I really cannot see it being especially warm at low temps in a t-shirt.
Overall a reflective bivvy is better than a non-reflective bivvy, all other things being equal but it's overrated when pushed up against things.
Yes loft concerns are universal but most bivvy bags are designed with that in mind as they're designed for use with sleeping bags but you're right, some of them are too tight and suffer because of it. If this one turns out to be as large as standard bivvies then it would be fine. But the video I watched made it look small on a little 5'8" guy.
EDIT: I've been looking up what proportion of bodyheat is lost via radiation and it turns out it's not a simple answer. It depends on the relative temps, air movement, perspiration, clothing etc. According to one medical paper (which is then quoted blindly all over the net) about 60% of heat is lost via radiation in patients but are they measuring surgery patients naked and lying still in a warm environment with little air movement? Looks like clothing reduces radiative losses quite a lot so wrapping in clothing with wind whistling around would be a different matter. But it could be higher than I thought, in which case the reflective nature could be more valuable - if the conditions allow reflection (air gap). If you get one, do report back how well it works - I'd be well interested.
