The RX/R designation doesn't actually say much about the underlying architecture, Polaris is GCN 4, while HD 7000 is ... a mixture. Most HD 7000 cards are first gen GCN, though some are TeraScale 1/2, which would required very different driver support.
I guess it doesn't "matter" to consumers buying a product, but it is unfortunate how confusing the naming schemes are versus the underlying architecture. The R9/R8 cards were similarly a mixture of GCN 1, 2, 3, and even TeraScale.
(similarly, the RX 5xx/4xx series has GCN 1 and GCN 3 cards mixed in, in addition to the actual GCN 4 chips)
It's really annoying because what you said is correct most of the time, and broadly speaking "HD 7000 is southern isles".
Except for the ones that aren't. And it's not even chronological -- the HD 7510 with a TeraScale 2 chip was released after some southern isles cards.
And there's the Radeon HD 7790, which has a GCN 2 sea isles chip.
I became aware of this because my "R9 280x" turned out to essentially be an upclocked HD 7970, using a Tahiti/southern isles chip. GPU was fine, it's just a nuisance to sift through what's what...
I guess it doesn't "matter" to consumers buying a product, but it is unfortunate how confusing the naming schemes are versus the underlying architecture. The R9/R8 cards were similarly a mixture of GCN 1, 2, 3, and even TeraScale.
(similarly, the RX 5xx/4xx series has GCN 1 and GCN 3 cards mixed in, in addition to the actual GCN 4 chips)