I think that once you get longer than about 5-6 ft, you're better off going with servos when using ballscrews. Servos have a couple advantages when using high lead ballscrews.
One, their high rpm allows you to use a belt reduction, which minimizes the affect that the much higher inertia of the screw will have on acceleration.
And you also get much higher resolutions, which you lose when using steppers with high leads screws.
I would think that a 2525 ballscrew would be fine up to about 6ft and 1000rpm (1000ipm), but I'd probably want to use either a spinning nut, or a larger screw for an 8ft machine. Remember, for an 8ft machine, you'd typically need at least as 9'-10' screw.
Unfortunately, 2525 seems to be the largest commonly available chinese screws. Anything larger, and the price tends to go up by about 5x.