The ACME screws are certainly an upgrade from the threaded rods. They give you a more accurate Z compared to the thread rods. Given how much the thread rods are bent, the improvement is anywhere between very little and quite a bit. The difference shows up mainly in "banding" in the Z axis. They also speed up the Z axis, that (to me) isn't a big deal. Your prints will come out 2 seconds faster ... The final improvement is in the durability of the Z drive. The thread rods *eventually* will chew through the nuts in the traps. As they chew through, things loosen up a bit. The Acme screws do the same thing on their nuts, they just do it slower ...