No, it only updates when a user reaches _exactly_ the given amount of posts. I believe I had two reasons for this, firstly it is the way threads handles title upgrades and I thought it would be a good idea to keep it consistent. The other reason was that it would mean that you could also move users out of the given group when they upgrade to the next level etc.
If you want to add the user to the group if they have as many as or more posts than the given number, you could make the following changes in the addpost.php code after you've added the hack:
Find this row:
if ( ($Totalposts == $posts) && !strstr($CurrGroups, $thegroup) ) {
Change it into this:
<br />if ( ($Totalposts >= $posts) && !strstr($CurrGroups, $thegroup) ) {
This should add the users to all groups they have enough posts for (although it is untested).
If you only want the highest group, you would probably have to do something like this instead:
Find these lines:
<br /> if ( ($Totalposts == $posts) && !strstr($CurrGroups, $thegroup) ) {<br /> $NewGroups .= "$thegroup-";<br /> break;<br /> }<br />
Change them into this:
<br /> if ( ($Totalposts >= $posts) && !strstr($CurrGroups, $thegroup) ) {<br /> $NewGroups = "$thegroup-";<br /> break;<br /> }<br />
This is also untested, but I think it should work.