Acf,
for the adding on a seperate line just search for:
$memsettings[6] .= ' (' . round($memsettings[6] / ((time() - $memsettings[14]) / 86400), 2) . ' ' . $txt['postsper_day'] . ')';
in profile.php and replace it with this:
$memsettings[6] .= ' </font></td></tr><tr><td><font size="2"><b>Post per day:</b> ' . round($memsettings[6] / ((time() - $memsettings[14]) / 86400), 2) . ' ';
This is a very lazy way to do it and I haven't tested it but I think it should work fine
As for the thing about putting it in display. Again this is VERY soddy but look for:
$postinfo = $txt[26] . ': ' . $psts . '<br />';
and replace with:
$postinfo = $txt[26] . ': ' . $psts . '<br />Post per day: ' . round($psts / ((time() - $userprofile[$mname]['dateRegistered']) / 86400), 2) . ' ';
Like I said I can't guarantee that any of this will work and it's VERY slack but it *should*do what you asked :|