Hot on the heels of the RX560 LITE launch, Sapphire also snuck a slightly upgraded RX550 on to their product stack, based on "Polaris 21" silicon, rather than the Polaris 20 silicon, that the RX 550 is normally built on. The Pulse Radeon RX 550 2GD5/4GD5 (model: 11268-16 for 2 GB and 11268-15 for 4 GB) has 10 out of 16 compute units physically present on the chip, which translates to 640 stream processors, higher than the 512 stream processors the RX 550 is originally designed to have. The card is running the core at a 1071 MHz core clock and stays within the 75W limit of the PCIe bus, with no extra power inputs.
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More here.