The thing about 54, 56 and 58 frames, is that most manufacturers make a significant geometry/sizing leap between two of those sizes, but not the other. For some brands, the 56 and 58 are within .5cm in stack with the same reach, while the 54 might have a 2.5cm shorter stack. Often, there are marginal differences between 54 and 56, while it's the 58 model that has significantly higher stack. Depending on how close to the frame's limitations your saddle to bar drop needs are, some brands may not be able to provide an acceptable range. The nice thing is, that if you need a significantly larger drop, and your current reach is correct, you have a nice range of stem sizes that will allow for some flexibility in reach, since you can compensate for a shorter frame reach with a longer stem to a point.

How much drop do you have currently? With what stem angle/how many spacers?

How much are you predicting you will need?