Indeed, 2006 was the first year that the MH chassis got the new cylinder heads that boosted horsepower and torque.
As one thats owned both an older V10 and now one with the newer V10, I will say that the difference in power is very noticeable, too.
Ford actually came out with the more powerful V10 in 2005. Their F250 and F350 pickup trucks got the more powerful V10 starting with 2005 models. The new V10 didn't make its way into the F53 MH chassis until a year later, in 2006.