I don't think it will increase your microfauna population too much, if at all(depending on how it's designed). The journey through the pump on the way back to the tank would destroy much of it. Some may survive though. Fuges designed to feed the tank through gravity, whether it's a DIY tank/fuge or HOB
type fuge, are best for this.
The benefit you would see is more water volume. That means greater stability. There would be less fluctuations in water chemistry, and also PWC
's may not need to be done as often. Also having a larger area to grow macro algae in would allow greater export of nutrients. That, combined w/ greater water volume, means you could add more fish to the display, assuming aggression will not be an issue. HTH