Hmm...seems like it should work. The reason it doesn't show at all is because the plugin unsets the $similarthreads variable so that it won't show in the usual place. So the str_replace isn't working - are you sure you put it in the same template you're using (postbit or postbit_legacy), and in the same style that you're viewing? Try doing something like this:
Code:
<if condition="$post['isfirstshown']">x<!-- similar threads --></if>
and see if the x shows up at all.