Not a K2CNC machine, but...
I've got a cheap Chinese router with ball screws. It sitting behind a Smooth Stepper and a G540 with whatever those 382 oz motors Ahren sells are on the screws. After I put in the SS I tested to well over 600IPM, but had some reliability issues with position during torture testing. I turned it down to 300IPM on X & Y and 150IPM on Z and it seems to be pretty good, but I have not run any long jobs on it since. I do have accell/decell set fairly conservatively, and it takes some distance to get upto that 300 IPM top speed.
To be fair though doesn't the K2CNC come with acme screws and Dumpster CNC style acetal antibacklash nuts.
I have a small box way mill with similar leads (retrofit) and the fastest I could run it without stalling or binding was about 80 IPM. (Straight parallel port no Smooth Stepper)
Bob La Londe
http://www.YumaBassMan.com