Hi Jon,
I'm currently building a Joes Hybrid R&P 8ft x 4ft
my build is budget constrained so that's why I went with the cnc routerparts pinion drives but even for my machine at 8ft you can go ballscrews- they just need to be 25mm or bigger I was advised to stop the whipping.
If you are still thinking of the pinion drives then please be aware that the spur gear they use is a US imperial pitch size, so if you don't have any machining options they could be very expensive to change to metric- I was quoted £100 each spur! I found it difficult to obtain the Imperial Rack size here so had to sort it a different route.
Afraid that's my only input as I have followed most of the plans fairly closely.
Best of luck with the build :)
Regards
Graham