I think the issue with Chief and the Evolution/MVVS 80 is Horizon was the supplier and has dropped the engines. Chief has no remaining Evolution 80 engines in stock. SInce Chief is an MVVS supplier perhaps they will eventually stock the 80.
In the meantime, I agree with your assessment of the 80GX. I have about 1 hour of run time on the engine and so far it has performed very well. The 80 GX is very powerful and incredibly smooth for a large single cylinder model engine.