For our manual overclocking tests, we used AMD's built-in performance tuning tools. Our best results are as below.
RX 6650 XT
RX 6750 XT
RX 6950 XT
All three overclocks resulted in pretty sizeable increases to the average clock speed of the cards in question, though most impressive is the RX 6950 XT, which ran almost 300MHz faster than it did out of the box.
These overclocks resulted in pretty similar-sized performance gains, where we saw typically an extra 5-8% depending on the title. Again, it's not nothing, and to be honest I was slightly surprised there was even this much headroom left, though of course this will vary from card to card.