I am going to expound on the 'Mejcanisimo' nature of the subject in my answer. I can tell that too many answers become contradictory in the end as the answers are taken from books written by Authors of differing opinions. This may be OK for someone who does not live the life of a Mexican every day on the border, which is to say that you cannot get any more Mexican than the border. Books are fine, however they completely fail to capture the true essence of the subject. The proper use in Mexican is that 'isimo' refers to the essence of something and expresses the degree of how you feel. 'Muchisimo Gracias' is a way of politely expressing thankfulness of a greater degree. It expresses a feeling and not particularly thought.
Hermano Gonzalo, you rely too much on grammatical rules taken from books and not from real life. Your word 'ciertisimo' may follow your books rule however, it is not a real word. The word Truth is truth in Spanish whether in Spain or Mexico and truth has no varying degree as lack of truth is a lie. That makes it a closed end statement or question, one where the question or statement is yes or no so truth cannot have differing degrees, sorry if I offend you. So when daily life does not hold true to books, is it not real? Books are written to educate and use as a guide, but the rules cannot be used in all real instances depending on the word and in the context of which it is used. Anything that is absolute in nature of context, cannot vary in any degree, therefore the correct rule of thumb would be to ascertain whether the word has a value of being absolute, if so, then it cannot be exponentially uncreased in any way, regardless of what any book says. Keep in mind that many books are written by linguistic majors who have learned from someone else's book and then write their own take on other's books. I do NOT own a single book on Spanish and only teach from verifiable usages through real life experience. So if I seem abrasive, forgive me I AM MEXICAN and can only give real life experience of over half a century of living on the border of Mexico and living the Mexican lifestyle in real-time. Hasta luego compadres.