He is actually correct, and I didn't know it either but apparently with the R9 290/390 you can mine at 30MH/s again if you do it with certain drivers, OS, and mining software.

I had a similiar issue where I used to mine 30MH/s with my R9 290 since 2016 and in 2018 it started to slow down to like 20MH/s before I shut it off. I assumed it was some DAG thrashing bug and just unplugged it.

However there is another thread going on in this category of mining where people were able to mine with their Hawaii GPUs again at 30MHs. However you can't do it on Windows. So search the forums if you need instructions on how to do it.

But keep in mind the GPU will use at least 200 Watts of power.