From my discussions with two speech therapists a few years ago, the most significant difference between early- and late-talkers according to current research is engagement (citation needed). Baby learn by copying behavior, including speech - the more they are spoken (and listened) to, the easier it is. This is different to e.g. the way babies discover a sense of "self" vs. "outside" world, which can be influenced by binding in other senses to their movement (there are some hilarious videos of babys that have some helium balloons on their hands and feet and discover, that they can move them - the more common alternative is hanging toys that make a sound when hit over the babies head).

Babies learn very quickly about their ability to produce sound, even before they get a feeling of "physical self boundary", so I'd guess the talk box wouldn't help much there. Talking and listening to the baby is the best bet to get them interested in speaking in general, and then giving extra positive feedback for identifiers like mom or dad or some other word for a physical thing.

My guess is, that one of those high-pitched speak-back toys would give better results on speech development than a talk box - while obviously not trying to underestimate the fun they (and their family) can have with the talk box ;)