As far as the whitespace is concerned in default.css the #main div has padding-top:66px and the .entry-content div has padding-top: 22px. I think those declarations are what you want to adjust in your childtheme's style.css to get your desired effect.
Regarding removing the page/post titles entirely, I think that it does more harm to SEO than good for design especially since I believe the undesired 'whitespace' can be dealt with via css. It's just a little tricky to see how the other elements work to create padding around the titles in the default design.
I wasnt sure what you meant by 'Pages' so here is how you'd go about removing the page titles entirely on WordPress Pages.
function child_remove_pagetitles() {
// Make changes to the original function
if (is_page()) {
$posttitle = '';
} elseif (is_single()) {
$posttitle = '<h1 class="entry-title">' . get_the_title() . "</h1>\n";
//continue with original function
} elseif (is_404()) {
$posttitle = '<h1 class="entry-title">' . __('Not Found', 'thematic') . "</h1>\n";
} else {
$posttitle = '<h2 class="entry-title"><a href="';
$posttitle .= get_permalink();
$posttitle .= '" title="';
$posttitle .= __('Permalink to ', 'thematic') . the_title_attribute('echo=0');
$posttitle .= '" rel="bookmark">';
$posttitle .= get_the_title();
$posttitle .= "</a></h2>\n";
}
return $posttitle;
}
add_filter('thematic_postheader_posttitle' ,'child_remove_pagetitles')
You can change the conditional to suit your needs adding or removing markup for titles on templates other than the page template.