Seems like a very average growth rate to me.They can do more or less depending on a number of things. One must always remember that all fish are individuals and growth rates on individuals will always vary from one another.
My 2 cents: 2.5"/month would be a super growth rate if the fish could sustain it and average 2.5"/month the whole year and reach 2.5 feet on its first birthday. Since monthly/weekly/daily growth varies, say from 0.5" in a slow month and 3"-4" in a fast month, it usually averages out just below 2" a month even for those keepers who are chasing the champion growth.
When thinking of growth, one should think longer term, not "instant" growth, because the latter can vary much. Some teens can add 6" in half year to their height - does not mean they will be grow up to be a towering NBA center in 3 years