About potting irises... I think it may not matter so much about the pot specs.
No doubt your growing zone probably impacts that. However,
I've had several of mine in a sort of "second class housing" situation for a few years. Wide pots, tiny pots, standard pots...
Old tidy cat litter bins with holes filled in the bottom actually work really well for TBs, in my experience.
I feel a bit guilty looking at this photo actually. They've been there a bit longer than planned.