Going up in ram speeds provides a great improvement on AMD Ryzen cpus. You will see a big performance jump going up in speed 2133-2400-2666 untill you get to 3000mhz. Going to 3200 from 3000 will deliver better performance, but not nearly as much as going from 2666 to 3000. I agree after 3200mhz there is little improvement of amd or intel based systems, partually due to the increased cas latency. I also agree only AMD currently knows how 3rd gen ryzen scales with increasing ram frequency, and how good the internal memory controller will be at those higher speeds. I would suspect simmilar scaling behavior as ryzen 2000 since the memory controller in 3000 is still based on 14nm.
Conclusion: If 3000mhz ram is much cheaper than 3200mhz, 3000 offers close enough performance to 3200 that i would buy the 3000. You are better off getting 3000mhz ram if its priced well and spending the saved money on a steam game or a new mouse or something.