yes there were some performance issues with the 8750.They probably had problems with the Z8750 but I guess they fixed it.
watch this: https://www.youtube.com/watch?v=Gf00PT69txg
//Sent from my glorious OnePlus X
I have an original z8750 and it's always worked fine.
If you ask me, the z8750 never had any issues. What probably happened was the review units were z8700s and due to their small number, they were probably all functional. Then when they did the first public batch with z8750s, they had the graphic driver crashing issue. In a hasteful attempt to fix the issue, they blamed the z8750 and swapped the z8700s back in.
Eventually, GPD realized they were still having the issues with the z8700s and silently tried to source the actual cause of the problem. Once they found the issue they publicized it and then proceeded to revise the GPD and gave it the z8750 back after using up any stock of the old one.
Of course this is simply speculation on my part, but it would explain a lot.