Do the barspells cap out (with Enhancing) as well? If so, at what amount?
I would say that part of that is the Red Mage Resist Petrify trait (WHM/RDM, depite having lower Barspell numbers still resists quite a bit with just Barstonra up, but it is still not a good reason to go /RDM since that doesn't help the group), but I certainly see the impact of Barstonra on doing that fight, I'm still not sure if Barpetra actually helps with anything but duration if it does stick however - although if I can I cast it anyways as part of dealing with all the dispel effects Tunga throws out.
I'll still hold my doubts that barailment spells actually do anything useful (shortening duration only barely counts as useful in select situations) but with the amount of resist you can throw out with a barelement, you can resist ailments quite nicely despite those barailment spells being useless.
Edit: I just noticed a post in the Japanese forums that seems to be about skill being useless after a certain cap, such as 500 Summoning Magic and 500 Enhancing Magic. I cannot really tell what SE's official reply was due to the automated translation but I think it is safe to assume that based on experiences here and in those forums as well that 500 Enhancing Magic may be the current cap for Boost and Barspells.