[boxbible]

Of course shadowboxing with dumb bells will increase speed and power. But is it a safe and efficient method?

When the weights are too heavy, it becomes injurious. If you can't keep increasing the weights, then you can only increase speed and power up to a limit.

There are tons of other stuff you can do that are far far better than shadowboxing with dumbbells. Put it this way. Shadowboxing with dumb bells might be good for beginners.