Short answer: there doesn't seem to be a cap. A very long time ago someone tried a UV Normal Max on a 5* plate shield and found that it defended slightly better than a UV Normal Low shield.
In any case, I don't think that any defense really has a cap. To keep people from getting infinite defense we instead have diminishing returns. I guess the theoretical cap would be when any more defense would produce an unmeasurable increase in abilities. A Max UV on plate doesn't reach that point though.
Because it already has so much normal defense, any more normal defense could be unnoticeable. It doesn't matter much whether it has a UV or not, so you can do what you like and feel confident in the decision.
Short answer: there doesn't seem to be a cap. A very long time ago someone tried a UV Normal Max on a 5* plate shield and found that it defended slightly better than a UV Normal Low shield.
In any case, I don't think that any defense really has a cap. To keep people from getting infinite defense we instead have diminishing returns. I guess the theoretical cap would be when any more defense would produce an unmeasurable increase in abilities. A Max UV on plate doesn't reach that point though.
Because it already has so much normal defense, any more normal defense could be unnoticeable. It doesn't matter much whether it has a UV or not, so you can do what you like and feel confident in the decision.