iMac 5k internal SSD vs external SSD performance
Because the maximum throughput of any port on the iMac 5k Late 2015 is only 10Gbps (Thunderbolt 2), no external storage solution can match the internal SSD's speeds.

The short answer is that you would give up a lot of performance.
The internal SSD can transfer data at about a couple GB/s.
An external Samsung 850 Pro 512GB via a self-powered USB 3.0 docking station runs at about 400MB/s during CrystalDiskMark sequential transfer benchmark on an exFAT partition.  

That said, for running macOS and applications, the difference between the internal SSD and an external SSD is much less noticeable than that between an SSD and a traditional HDD. Unless you are constantly moving large files on and off your drive, an external SSD won't feel much slower subjectively than the internal SSD.

You did not specify how much data you have or your budget, so I cannot give specific recommendations. But in general, I put macOS and applications on an internal SSD. I would figure out how many GBs macOS and all my applications would consume, and choose an SSD of appropriate size.  

If you really value the drive being internal and need large capacity on a budget, consider the 2TB or 3TB Fusion Drive. They have a 128GB SSD built in and intelligently find and put the data you access the most frequently on the SSD portion. The 1TB Fusion Drive only has a 24GB SSD so is less attractive  

Thunderbolt 2 enclosures doesn't make much sense 

True, an SATA SSD could be bottlenecked by USB 3.0. But not significantly so. That is because SATA itself tops out at 6 Gbps (SATA3), not much more than USB 3.0's 5 Gbps. 

For the same reason, even though Thunderbolt 2 eliminates the link bottleneck, the benefit would be small because now the bottleneck is on the SSD side, which is 6 Gbps.

There are PCIe NVMe SSDs (the internal SSD falls into this category), which are faster than SATA SSDs. But to use those externally you still have two problems:


Thunderbolt 2 tops out at 10Gbps, slower than what the internal SSD and other PCIe NVMe SSDs are capable of. 
There are few (none?) Thunderbolt 2 to PCIe NVMe SSD enclosures. The StarTech product you linked to takes SATA drives. 


All Thunderbolt 2 enclosures I have come across are ridiculously expensive, including the one you linked to, especially considering the limited performance gains. You would be much better off spending the money on upgrading the internal SSD.