In past experiments on the original 100BaseT equipped AppleTV (which is why LE/OE have support for nearly all the chips involved; I tried them) moving to a USB gigabit adaptor lifted speeds from ~11MB/sec to a whopping ~14MB/sec. Why so little? .. because running networking over the USB stack requires a lot more computational overhead than using Ethernet, and this taxes the CPU and negates many of the theoretical gains (even at USB 2.0 speeds). I would be very surprised if anyone reports a significant user-experience improvement using one.
TL;DR .. cheap boxes with 100BaseT are cheap boxes with 100BaseT .. caveat emptor