The memory latency does not work like that. The chip read latency Is constant regardless of the interface clock frequency. Latency is measured in clock cycles, it is why ddr4 has higher latency measurements than ddr3. And the memory cells being used are no different to those in ddr5, just they are stacked etc.
Plus as I showed, the 1080p performance is fine when using mantle so the low 1080p performance is being caused elsewhere than the HBM.
I thought about it last night and I think the cause is 'Over aggressive memory management' in the driver. The memory managment is in the game for thief in mantle mode so will not be affected by amds driver memory management, causing lower res performance to tank.
If you look at game memory usage at lower resolutions, such as in gregs Pcars video, the FX is using half the ram of the TX yet there is plenty off memory to use.
You might be on to something here, a lot of buffering is being moved around to keep the VRam usage low, if its constantly flushing and uploading the pool it will cause a slowdown, if its moving objects between the GPU buffer and system RAM it will increase the latency massively.