The sub-pixel circular target marker is accurate to 0.05 of a pixel for very high-contrast, low-noise circular targets down to about 1 pixel accuracy for lower-contrast, noisier targets. Generally the LSM method is more accurate and especially on small targets.
The actual final improvement in the accuracy of your project is hard to determine because it depends on the number of sub-pixel targets, where they are located on the objects, and how well they appear in the images etc. Generally for a project that is mostly circular targets and done with a calibrated camera you can expect from five to twenty times improvement in accuracy. So for a three mega-pixel digital camera you could expect about 1 in 15,000 to 1 in 60,000 accuracy. For a project that is a mix of natural, human-marked points and some sub-pixel points you can expect from two to ten times improvement in accuracy for the sub-pixel marked points.