The model iMac you have has two Thunderbolt 3 ports. These two ports support DisplayPort, Thunderbolt (up to 40 Gbps), and USB 3.1 Generation 2 (up to 10 Gbps).
The SATA3 bus internally supports 6Gbps (750MBps) for internal drives, so theoretically the external drives might be faster.
But, the drive itself is SATA3, so regardless of the TB\USB3.1 speeds, the drive itself will be the bottleneck at a max of 6Gbps. USB3.0 speeds are roughly 5Gbps, so the upgraded dock would help (20% roughly) with transfer speeds between the Mac and Dock.
However, the drive itself supports less than 2Gbps sustained read or write speeds, which might be faster than HDD, but the bus, be it USB3, 3.1, or TB would be overkill. SSDs approach sustained read\write speeds of USB3.0.
So, faster transfers will not necessarily result in observable differences. The main difference would be SSHD VS HDD performance which is roughly 210 MBps (SSHD) vs 80-160 MBps (HDD). Some SSD by comparison can reach 500-550MBps speeds.
So, save some $$$ and put an SSHD in the existing USB3.0 dock.