You would CERTAINLY get faster speeds out of a USB3.0 device.
The write speeds are not dictated by rekordbox, but rather the device itself and the operating system. I use a SSD in a USB3.0 enclosure and it tears the bits and bytes off my "high-speed" USB2.0 device.
If you don't have a USB3.0 connection, you may find FireWire800 is faster than USB2.0 but ultimately, still not as fast as USB3.0. One thing you may consider is adding an ExpressCard with USB3.0 support.