Restricting to 4.8.1 would certainly be a bit annoying, are the bugs that major?
I don't believe so: remember that Boost.Config primarily exists to provide workarounds for what Boost libraries are actually doing. In that context I believe the macro to be set correctly - at least in the sense that library authors haven't been complaining that the current settup breaks their stuff.
How would I determine the severity of the bugs? The feature is either broken or incomplete and there are no workarounds, so it sounds major to me. My use case is somewhat contrived, I give you that, but that doesn't negate the fact that there is no true SFINAE for expressions support. Maybe this grants having a BOOST_NO_*FULL*_SFINAE_EXPR config macro, drawing some arbitrary line on how much conformance is "enough" conformance.
Ugh, please no. Is there a use case for this within Boost or is it an arbitrary test case? My gut feeling is that folks that need to do "complex" things can always use a BOOST_WORKAROUND instead? John.