You could spin the nut instead of the screw, that would eliminate whip and allow faster rapids. Check eBay for ballscrews, I'd come across many long ones for pretty cheap prices... Also have you looked at the CNC Router Parts r&p system? Lots of users here, and no need for split pinions. There are many ways to implement belts; check out the Solsylva design, Momus design (they have a forum here), Mike Everman's ServoBelt (thread here too); even the MechMate has a belt drive option.
In fact, it might be worth the $100 to get the MechMate plans, since all the figuring is done for you!