While it is true that TLC triples the capacity of a NAND chip. It does not 1/3rd the write speed. Or the read speed. It depends on how the NAND chip is built. You cannot just say that 8 * 1600 is the write speed of SLC chip so 8 * 1600 / 3 is the write speed of TLC flash.
I said it before, I'll say it again. 8 * 1600 is the speed of the connection between NAND chips and the controller. It is not the speed of SLC chip or TLC chip or whatever. It is like.. Windows shows your Ethernet connection operates at 1000 Mbps. That is not the speed of your internet because the speed of your internet is determined by your Internet Service Provider.
If you want an even simpler example, when your open the faucet of your shower in your bathroom, how much water comes out of it is determined by how much water your water supply is pumping to your house. If they are facing problems, the water will come slowly. If everything is working fine on their end, you get water at full speed. Your pipes are the same. But the speed/amount of water you get depends on the supply.
Similarly, 1600 MT/sec is the speed of the connection. Eight 1600 MT/sec connections are the 8 pipes. The actual speed (amount of water you get) is determined by the NAND chips (the supply) and how they are made. And going from TLC to SLC does not scale write speeds proportionally. It does not scale read speeds proportionally. There is not a single performance metric that is scaled proportionally.
Here the 970 Evo goes from 2400 MB/sec to 600 MB/sec. In my previous examples, the 980 Pro goes from 4000 MB/sec to 2000 MB/sec. WD's SN850 goes from 5000 MB/sec to 1000 MB/sec. They all scale from SLC to TLC differently because the NAND chips in them are all built differently.