Yes I can see the pro's for dropping mr Ra for more psionincs but do you have any of the other Phoenix Lords?
Jain Zar or Karandras would also be useful as a big bug killer.
Hmm, have you also thought about Warlocks to bolster the guardian units as well?
The Guardians have their Warlock. Mr Ra is the best option PL-wise against nids, he can lay down the fire that is needed and he is not far behind in CC. Tough taking a CC oriented PL versus Nids falls on the same hurdle as the Avatar. Even more so as they don't have any ++save. If I had had a vyper or two I would have taken formations. Windrider Host + Seer council + Aspect Host + Engines of Vaul. This is a list I would love to try out. Psychic tests on 3+. One use shread on the windriderhost, BS5 Dire Avengers and Reapers. But, what I have is what I have. If you want to know what I have you can have a look here