have your Gigabit one broke? if not I don't know what it will help to get that one, nothing will improve.
Real world max speed of single 7,200 rpm drive is about 80 MBps. However, Zidoo is based on Android which is not built to transfer large files at high speed. I have a Z10 Pro and put a SSD in it to play around and to store HT4 images in it. The max speed burst I got was around 120MBps (make sense since that is the max of gigabit connection) but the speed dropped down to about 40MBps average after the initial burst. So, 2.5gbps connection is unnecessary.
Assuming 256MB cache, it can perform at that speed for a whopping 0.30 seconds if my calculation is correct. After that the disc I/O become a bottleneck to repopulate the cache.
My Exos drive has an average speed of 211MB/s during parity check. This means reading the whole disk and takes around 23 hours for 18TB. The max is around 270 and min around 160 if I remember correctly. so you can definitely sustain greater than 1GBbit speeds on HDDs. Whether the Zidoo can deliver that performance is another question though. If you just want to use it for sharing the HDD I would recommended a proper NAS instead. Something from Synology or Qnap for example.