Actually, with GDDR5 it's really easy, you just solder chips to the back of the motherboard so they're connected to the chips on the front. This "clamshell" design is a unique feature of GDDR5 and requires no modification of the memory controller or motherboard layout.
but in this way the chip can transfer only two 16-bit data-words per clock cycle instead a two 32-bit, am I wrong?
The most probable theory is that they were already in clamshell mode, and they decided to use ram chip with twice the density (from 4Gbits to 8Gbits). This doesn't require any changes to anything. It's only a question of procurement and cost.
The only similar mod to durango would be clocking the SoC higher, and/or using 16GB.
reasonable, but those chip's are not available for mass production, from what I know..