4 Zen1 cores will obliterate 8 jaguar cores easily especially in single-threaded limited scenarios. clock them at their optimum around 2.5 ghz and they are very power efficient. I admit the gpu part would be somewhat of an issue. Vega 56-like could be around 450mm2 + 96mm2 for 4 zen1 cores = 550mm2 on 14nm. maybe bit smaller on 12nm. For sure doable in 2020 when yields should be excellent.
I remember a graph when zen was fist being tested. I think saying very efficient is if anything an understatement. I remember being shocked at how low the power usage was when it was clocked lower.