What is the fastest device to use to restore a timemachine date based backup on a mid 2015 mbp?
The fastest port on this machine seems to be the Thunderbolt 2 interface at 20Gbps. Are there any drives that would get anywhere near that? 


Chances are very unlikely.  Why?  Several reasons:


Thunderbolt 2 has been deprecated in favor of Thunderbolt 3 since late 2015.  The market for these devices were small to begin with and is shrinking more quickly with each passing day.
You didn't say specifically what you're restoring from but suffice to say the commonly accepted media is an external hard drive. Assuming it is a SATA III drive, the throughput will be at most 6Gb/s 
Even if you were to connect two Macs with a Thunderbolt 2/3 port and get a full 20Gb/s , you're still limited by the speed of the drive on which you did the backup
There are no USB 3.1 (or what is now known as 3.2 Gen 2) adapters or devices in the market; they were never manufactured.  Why?  USB 3.1 was introduced alongside Thunderbolt 3.  The most you'll get is USB 3.0 (or 3.2 Gen 1); 5Gb/s throughput.


The fastest devices in order would be the following:


Thunderbolt to SATA enclosure like the Lacie Thunderbolt Drive or G-Drive.  The Thunderbolt 1/2 versions of these drives are discontinued so you'll have to find them on eBay or similar sites.  
USB 3.0 to SATA adapter or similar enclosure.  This will give you the 5Gb/s throughput; closest to the 6Gb/s SATA III throughput.



  Is a 3.1 USB thumb drive good enough or would a standard SATA3 SSD drive work faster?


Get it for the "future proofing" and get it for the fact it's solid state and less components to fail, but as for throughput, you'll never see it.