The (nearly) mythical Radeon R9 285X.
Finally answering questions that everyone was asking ~8 years ago: why is Tonga 384-bit, but the R9 285, 380, and 380X are 256-bit? Answer: it's a different chip!
Okay, well not actually a different piece of silicon, but AMD designed and manufactured two different package types for Tonga. One which became Tonga and Antigua PRO/XT with the 256-bit VRAM we know, and another which AMD called "Tonga T3" which exposes the entire 384-bits inside the ASIC. This technically
should have been Tonga XT, but for reasons only AMD knows it didn't. Tonga T3 is an interesting look at what Tonga was perhaps meant to be in AMD's product stack. This card was produced in the last half of September 2014, right after the R9 285 (Tonga PRO 256-bit) launched as a real product, and looks to be equipped to be a direct and immediate replacement for the Tahiti based R9 280X on all fronts. Same 2Gb density (256MB) 5.5Gbps GDDR5 traced out in a functionally identical 3GB array, a 1GHz core clock (albeit with no turbo functionality enabled), and a very effective dual-fan cooler that remains whisper quiet.
All of the GCN3 features such as delta color compression and enhanced power tuning are enabled and functioning, the card shows no signs of being terribly early in development, in fact looking more or less complete, and the drivers pick it up right away which allows game and benchmarks to work without a hitch. So why didn't we see this thing launch? My theory is pricing, and a lack of any real niche to fit in. Here's what I think happened: Tahiti (as the 280X) is selling well in 2014. It's been clock bumped, outfitted with new designs, and it's affordable having been on the market for nearly a year at this point. The legendary status of Tahiti as an overclocking monster makes it popular with enthusiasts, and its now more mature drivers make it popular with gamers. It's also shockingly not much slower than Tonga, albeit with reduced features and nearly a third higher power draw at the same clock. AMD has the R9 285X in the labs, they're looking at how the R9 280X is performing at under $300, they're looking at the BOM (bill of materials) cost to produce either Tonga and replace Tahiti for good, or just keep production on Tahiti going, and they're looking at 2015 when they expect to have the big GCN3 (Fiji) rolled out. Tonga in its 384-bit incarnation essentially uses every component Tahiti does, with the only major savings being on power delivery components. So it's not going to be THAT much cheaper to produce, and they've already got this 256-bit Tonga PRO developed alongside that is going to reduce packaging, VRAM, power delivery, and board design costs. Tonga T3 also does not significantly outperform Tonga 256-bit due to Delta Color Compression exceeding expectations in VRAM bandwidth savings.
So they cut Tonga T3 from the stack, release 256-bit 28CU Tonga in 2014 to show they really are delivering on their roadmap, and slate a revival of Tonga in 2015 to coincide with their biggest investment to date, Fiji, which shares the GCN3 ISA and features with Tonga. By 2015 GDDR5 prices have come down and they can even equip the new cards with double the capacity, making cards like the R9 380 and R9 380X much more competitive. It also sets the development stage for Polaris, which at a deep architectural level is just Tonga ported to 14nm FinFET and with more fine tuned features to really drive up efficiency.
Tonga T3 ends up some kind of awkward cryptid in the end. Developed, built, tested, rumored, hyped, cancelled. Leaving behind echoes of, "But what if?" and, "But why?" questions on internet forums and message boards.
To date it's only been verified that three of these cards still exist, but my experience with AMD samples gives me the impression that there were likely a few dozen that made it to this stage of development.