As always I will defer specific advice to Matt and David and restrict my comments to my personal experience.
You ask a great question, how should one decide when to move up in level, and are you better served by adding strength work or milage to your efforts?
I am currently following a modified (by me) maintenance plan. By modified I mean that I am taking the lessons learned from my recent experience with the Stride Academy plan and altering workouts to include cadence and shoe cue work and adding a stride and drill workout to the stock maintenance plan. In addition I am following the strength program and have formalized my mobility work by including it on my Training Peaks calendar.
These additions add several hours of work each week and taking the extras into a Level III plan would be a burden, but for an aging athlete such as myself I feel I am getting better results than additional milage alone can deliver. I suspect for a younger athlete it would be better to follow the plans as developed since they are designed to get you to the best possible running form in the shortest possible time.