From my own observation and what some older birds tell me, it is a gradual curve instead of yes/no increase. For e.g.slightly above mid point then a C+ can in theory get some increase although this will be lower then normal, but as you get nearer to cap you might need a B+ or more to get the same small increase.

Let's say you are the typical C- to B- range of performance, there will be no increase when you are close to the cap. However every 3-5 years, MOE will decide to increase the range for each level, so if you happen to catch the year when they increase you might get a little increase for 1-2 years before nothing again.