I think my pots are around 15-18 cm. I still have some varieties in a 7-8 cm pot, but kohlerias do better in a bigger pot for me. They started to produce more rhizomes and more shoots.
I think, to see the difference you will have to grow and have them in bloom at the same time. Maybe Lola can tell us, because she had them once blooming together.
Have you noticed that this variety is chameleon? You can clearly see which flower is older and which is younger. The younger ones have bicolored background, and with the time it's turning into single colored.