Are the Boeing 747-8 engines more efficient than the 747-400 engines?
Yes. According to GE, the GEnx-2B consumes 15% less fuel than the CF6, which was contemporary to the PW4084 on the 747-400.

The bypass ratio of a jet engine makes it more efficient, not less. A higher bypass ratio provides more thrust from the fan air, which means the core section can use less air and therefore less fuel to provide less thrust. This increase in efficiency is the reason turbojets evolved into turbofans and have been increasing in bypass ratio as technology improves.

One consequence of a higher bypass ratio means the engine does get larger and heavier. However, the increase in efficiency is more than enough to offset these costs.

Besides increasing the bypass ratio, there have been many internal improvements in the GEnx line of engines, discussed in the GE link above. These changes include composite fan blades and fan case, which can be lighter than the metal alternatives. Throughout the engine there are also aerodynamic improvements to increase the efficiency of air flow through the engine, as well as more new materials and manufacturing technology.