Smaller gaps (high coupling) between filter resonators decrease the loaded-Q of the resonators.
This will affect the performances of the filter as: in-band ripple and insertion loss, and out-band rejection.
Usually in a filter design, the first and the last resonators (input and output) have tighter coupling (smaller gaps) with the adjacent resonators.
When reaching the lowest limits of these gaps, also have to take in consideration the voltages (powers) going into to the filter.