If you do not have enough knowledge to understand my answer, Im affraid, answer to your question will not tell you anything more.I'm not clued up enough on this topic in order to be able to interpret your answer. How much faster, compared to the GPU in the upcoming MBP, would you say mobile GPUs will get over the next 2-4 years?
Simplest answer: Only AMD knows. You can extrapolate from the benchmarks, and percentages. If Vega Pro 20 is 60% faster than Radeon Pro 560X from current MBP, you can expect that higher core count GPU with higher core clocks, and more memory bandwidth will also be much faster than RP 560X.