> we are limited to northern island and lower cards.

It's more complicated than that because the actual GPU architectures, which are what determine (non-)support, don't correspond to the "Islands" family names (which correspond to the R(V)Xxx GPU names). The Northern Islands family (= RV9xx GPU) also contains such GPUs with TeraScale 3 architecture (i.e. HD69x0 cards with RV970 "Cayman" GPU) that isn't really supported by MorphOS.
In conclusion, it would be more correct to say that MorphOS support is limited to most of Northern Islands (i.e. RV910 "Caicos" GPU to RV940 "Barts" GPU) and all lower GPUs.