Relation between ionic resonance energy and polarity
I'm not too familiar with resonance theory, but I'll try:

According to modern VBT, the general idea is that the state of the molecule is not described by a single state wave function, but rather a linear combination ("superposition" if you will) of multiple different state wave functions. In your particular case, the resonance is between a "dominant" covalent structure (involving a completely equal sharing of electron density) and a "minor contributor" ionic structure (with unequal electron density). 

Since resonance energy is defined as the amount of energy needed to convert the true delocalized structure into that of the most stable contributing structure (which can be interpreted as the amount of stabilisation provided by the linear combination of the state wave functions), a higher resonance energy would mean a greater contribution (and hence weighing coefficient) from the ionic structure. This means that there is a greater ionic character to the bond (and it is more polar). 

Hope this helped.