I don't like the tooltip in general, since it justs lists the first post of a topic again and again, but I do like it in the search results.
I just turned of the tooltip, and added one line of code to /scripts/dosearch.php
Find
extract($in, EXTR_OVERWRITE | EXTR_REFS); // quick and dirty fix - extract hash values as vars
add below:
$config['TOOLTIP_LENGTH']="300";
Adjust the 300 to whatever length of tooltip you want.