Yes, you lose a significant amount of heat via radiation.
A full radiant barrier wouldn't be breathable. Some companies have been playing around w/partial radiant stuff in clothes (shells from GoLite come to mind), but not sure how it would work in sleeping bags.
An emergency bag liner used as a VBL can easily add 15 degrees or more to a bag. You would need many more ounces of down to achieve the same result.
The VBL liner must be used as a liner, not as an overbag. If it were over, or used as part of the outer shell, then the insulation would get wet.
VBLs/radiant barriers can add significant warmth; I've used them regularly in temps as high as the 40s. I've only ever been warmer. They can't really make you colder if used properly. Most people would recommend that you don't use them above freezing.
Warmlite does use such technology in their sleeping bags, and has for decades. They aren't exactly seen as mainstream or as using mainstream philosophies, though.
Most people aren't comfortable w/VBL and find it harder to regulate their body temperature. I suspect that w/radiant barriers it could be difficult to manufacture an aluminized fabric with the same lifespan as a typical lightweight nylon.
Using a reflective blanket independently of the bag adds flexibility to your system. If you're cold, you can toss the 1.5oz liner in and add 15*F of warmth. If you're warm, don't bother.