[kong]

Lol, 400IPM rapids on a mini lathe, I wonder how long the bed would last!
Seriously though, I am using Mach2 to control the lathe, with the aid of a tachometer fed into the software, cnc threading is possible. Mach2 will adapt the feed speed to maintain the proper pitch on the thread. Of course, I have not got that far yet since I have had no need to turn threads, but you may be able to use the info so the servo does not have to drive the spindle too.
As for my x-axis, I am using an unknown servo motor I picked up on ebay for about £5. It is a slow speed type - about 500 rpm, but bags of torque. I run it 1:1 ratio, and have never had it stall, even though I have some accidents with the work being pulled from the chuck. I guess 100 oz/in would be fine, our little lathes don't cut that fast, so you should be able to maintain the torque. Obviously, if you can get a more powerful one, it will be a bit of insurance against lost steps if you are going the stepper route.