suggested by Kliment via IRC (/ht), the way to have a heavy toolhead moving about in 3D with high speed AND precision is to modify a ShopBot instead of a Rostock. Recall, the Darwin suffered this design challenge which led to the Mendel.
With RAMBo bypassing the stock motherboard we can drive the ShopBot to scary speeds (10x faster in XY and 100x faster in Z.). Precision should also be ~10x better than belt-driven motion, but needs more fine tuning.
and did i mention it was freaking awesome?
My ShopBot RAMBo Marlin firmware branch is available via GitHub (of course). Follow along in the GitHub log to understand our process.
Thanks to ShopBot and Ultimachine for all your help and schematics!!