I don't know about this particular variety, but if you are fertilizing and controlling disease, and it still doesn't produce, I would topwork or remove an unproductive tree. I'm not sure how many trees you have, but mangoes can take up a decent chunk of real estate, that can be used for many other productive things.
I would also say 5-6 years is a fair assessment of a variety in most cases. Some people said they had varieties like Coconut Cream take almost 10 years, but by then most 10 year old mango trees are very big. I've heard similar things about Sugarloaf- another highly regarded cultivar lately. taste/production balance.
Just some stuff to consider