I think 800Z looks better rated at 640, especially in low light.
For what it's worth, I think the two grainy shots look great; a really nice 'peppery' texture to them. I wonder if a soft filter would have added to the feel.
I'll take your remaining 800Z off your hands, Ted, if you do decide to get rid of them. 800 speed film is my favourite, but I never shoot in low light.