Does it work with 1.5? Yes
How many queries?
-1 query on dedicated page
-If widgets are enabled its 1 query
Widget Framework
We initially wanted to support a wide audience, so created a fallback for non-widget framework users as well. For you to have it display the box 1 time, you'll have to go inside the Options->Top Poster Of month and then uncheck those two checkboxes there which says whether to show the leaderboard and previous month winner or not.
This will hide the default fallback option from showing and you'll have only 1 set on the site.