Encoder ppr?

How fast do the servos rotate? If you use the 7i77 then I think you can count pulses at 10MHz.

Also calculate what spatial resolution your encoders give you. Much less than the width of an atom and you are wasting money

I would say that the more resolution you have the better, up to the point that you overload the counter.
720 kHz is comfortable for the Mesa counters (I think they can go to 10MHz with the filters turned off)

What sort of accuracy are you hoping for? Your PID loop needs a few counts error to work with. 0.3um encoder resolution probably translates to no better than 5um positional accuracy. 1500 ppr would thus be about 10um accuracy. I would imagine that this is OK for a mill, good for a router and a bit worse than you would hope for on a lathe.

You might want to consider a belt reduction between the motor and the ballscrew. You can gain resolution and force at the expense of speed. 3000rpm and 5mm pitch is a top speed of 250mm/sec. For a big router that might seem slow, for a small lathe it might seem fast.