A couple things here.
One, by the time you hit your late teens, the number of brain cells you have are going to have to last you the rest of your life. As you lose them, your body doesn't make new ones. Gross oversimplification, but for the most part true.
Second, even just a little boxing will hurt you. Through the 1980's and early 1990's, the US Army conducted some of the most comprehensive cohort studies of the health impacts of boxing. One of the more important focuses were the neurological effects. What it boiled down to was that almost every boxer suffered measurable declines in cognitive ability. When soldiers were tested and retested in 3 and 6 month intervals, almost every single subject showed a measurable drop in
g within a year, some within 3 months. These losses were statistically significant (p < .05). This is a pretty big deal, whether or not you believe in
g and IQ because the tests showed a relative loss for the same indiividual.
What was interesting was that the beginner level boxers generally suffered the most, in addition to sustaining a greater number of injuries in general. This was chalked up to these boxers being less experienced. The general model seemed to indicate a good bit of loss up front, followed by a less steep but still profound loss of cognitive ability over time.
The abstracts aren't up on PubMed, but those of you with access to academic or medical libraries should be able to dig up copies if you want the reference.
In short, if you box competitive, amateur OR pro, you WILL lose brain cells. You WILL get "dumber." You may not notice it because it happens over time, but it WILL happen.
On a personal note, MT and boxing are the two things I would not do again if I were given the chance. I know for a fact that I suffered a sharp drop in my intelligence in the course of a year, only noticable because I was in college at the time. I could go on about exactly the type of things I noticed but its actually kinda depressing.