[XeTeX] Comic Sans and mboxes in xelatex

[nachman]

Folks,

I use Comic Sans at 24.88pt in XeLaTeX in conjunction with Keynote  
for math lectures.  I use Comic Sans in math expressions like

     $\vec{\mbox{n}} = (a,b)$ or  ${\stackrel{\longleftrightarrow} 
{\mbox{AB}}}$

Overlines and underlines are always way too high or to low.  It seems  
that the mbox for this and similar fonts has a lot of extra vertical  
space.  Is there a (relatively) simple way to get rid of the space  
above the letters so that vector smybols, etc., hug the top of the  
charcters as in plain vanilla LaTeX?

Jack Nachman