Definitely not worth it. You can see for yourself, even if there's 100% scaling (there won't be) :
RX 550 = 512 shaders , 16 ROPs, on 128 bit memory bus with 50W TDP
Let's say you double that, you get 1024 shaders , 32 ROPs, on 256 bit memory bus with 100W TDP which is weaker than a 2048 shader RX 570 8GB in every way because 1024 shaders is only RX 560 level which incidentally is a ~90W part and Crossfire doesn't double memory size.
I haven't mentioned TMUs since they're proportional to shaders for GCN architecture.
In China, AMD is launching the cut down RX 470D again as RX 560XT which will have 1792 shaders , 32 ROPs, on a 256 bit memory bus. That's probably a good option if you're in China, but for everyone else that's on a budget the RX 570 is the card to buy undoubtedly.
There's basically two cards worth buying in AMD's lineup if you have the patience to tune them: RX 570 (~120W-160W TDP BIOs) < $150 and RX Vega 56 (150W-165W TDP reference BIOs , Nitro+ is 180-240W limit) <$350. RX 580 / RX 590 are clocked to the wall so they're power hungry for their performance class , RX Vega 64 and Radeon VII aren't as price competitive as they could be and are also power hungry out of the box.