The parenthesis highlighting is a bit of an all-or-nothing proposition, since Vim can't really tell if you intended to leave unmatched parentheses. You can disable it altogether (which won't highlight any paren errors) with
:hi link arcParenError NONE
:hi link arcParenError Error
If you find any, feel free to let me know of code it doesn't highlight correctly: http://bitbucket.org/fallintothis/arc-vim/issues/