Got KSM and im done till at least 10.1 but my healer was 392 equipped (394 after reset) and I was doing 15-16s, im a R Druid though.
North of 400 should be very doable. Same affixes youre used to, same mechs that will probably 1-shot you if failed, mobs that were dangerous in 15-19 are more dangerous but you have higher stats and typically better players with a greater understanding of mechs to counter it, typically better comms rather than a silent group adjusting to other randomly assembled players with diff strats on the fly…
For example, I still had plenty of groups just facetanking the degenerate dot from the big water elementals before Wise Mari in TJS. That dot can be lined.