This is getting interesting.

I have a few observations,

1 PCM and the MPX IPD systems seem closely matched from the tests carried out by John above. ( No experience of IPD myself)

2 The Diversity system looks like you get Rx failure backup more that interference defense.

3 Where the Diversity system would provide a huge step forward in signal integrity would indeed be if they both were on different frequency's as Mike said above. My question is, How do we transmit on two different frequencies at the same time from the same Tx?