I'm not convinced it's bandwidth-starved.
I have a DDR4-3200 4700U at work and a DDR4-2400 2700U at home. I run Unigine Superposition at 1080pMed preset last night and got 1900 points. The 4700U just got 2050 points this morning.
As far as I'm concerned the poor old 2700U is damn close to the 4700U despite the 4700U having 50% more bandwidth. Whilst the LPDDR4X variants are faster, they're not
that much faster and if there was a absolute memory bottleneck, Vega8 would perform no better at all - but it's significantly better, even on DDR4-3200.
1) The memory bottleneck does play a
small role in the graphics performance, otherwise the LPDDR4X variants at the same cTDP would be no quicker at all. They are up to 10-15% faster for a dual-channel setup.
2) There is an
enormous amount of scope for improved performance over a vega7 using exactly the same DDR4-3200. The Vega8 in the 4800HS with higher power draw runs circles around the vega7 even though it's an apples-to-apples comparison with identical memory configs.
It's just CUs and how much power it's allowed to use. The old 2700U has 43% more GPU in it.