The current definition is (I believe)
#define BOOST_LOG(logger) \ if(!logger.isEnabled()) {} else logger
Which achieves the same ends. In particular, foo() is not called unless logging occurs.
But there is some additional overhead with the implementation in the proposed (and withdrawn) Logging library. The conditional actually creates a temporary object (enabled_logger) that contains a pointer to a heap-allocated stringstream (in the case of the log being enabled)
This is implementation detail of submitted library (mistake IMO) and should not affect overall preferance on interface. It is possible to implement filtering part efficiently without need for extra memory allocation and unnecessary work In my production projects in performance critical application I also use one global variable that guard all DEBUG trace statements. Gennadiy