Your machine is looking great! I'm slightly biased, of course, but it looks like you have a solid foundation to start adding to. It's great to see the linear carriages in action.
I wouldn't hesitate to recommend Dumpster's AB nuts -- they work really well on my machine. I get about 300 IPM rapids on the X and ~150 IPM on the Y with the 5 start screws and nuts. I have 300 oz-in motors, and you're close to that, so you'll hopefully get similar performance if your gantry mass is about the same.
Best of luck!