On the older chart is the slightly older 295x2 which was 2x 290x on one card. Similar but 390 is rebadged 290, so less cores. The chart is saying 295 was similar to GTX 1080, but that's best case scenario when CF is scaling properly. With the less shader units, it would probably be closer to like a FuryX, again when it's working right.
Upgrade from a 380? Meh. What's weird about that is that the 380 is a rebadged 285, aka Tonga, which is GCN 3rd gen. So while it's an upgrade in performance overall, it's one step backwards going from 3rd gen to 2nd gen, losing much efficiency. I know it's just horrible times to buy a graphics card, new or used. I watched my Vega 64 I bought used for $180 last year now goes for close to $1000 on ebay which is just insane to me. I'd sell some of my cards for cash but then I wouldn't be able to replace them with anything. I'm hoping some of the new 6000 Radeons get stock by summer, but I doubt it will happen or prices will still be crazy high.
My advice: hold what you got. Wait until you actually find a GOOD deal, whether it's new or used. You can tune game and driver settings to add more performance if you are lagging for now.