Well, the comm system has been upgraded too. Voyager introduced "concatenated coding", which quickly became very widely used in both space and terrestrial applications. Newer deep-space missions all use turbo coding, which wasn't invented until about 15 years after the Voyagers were launched.
Turbo coding reduces the energy you have to transmit per bit by nearly 3 decibels, which means a near doubling in the data rate that can be supported on a given power-limited radio link. And in deep space, they're all power limited.
Still, over the great distances involved it will take something like a year for New Horizons to send down all its encounter data. We'll have to be patient, especially for the really sexy pictures.
