Most TV systems (Projection, direct view, Plasma/LCD) overscan a lot. There are no easy work-arounds. The Video Wizard has an adjustment that adjusts LMCE's menu's but can't adjust the other sources. Scaling in the system could be done but it would be a long slow process of tweaking modelines and it may not work.
You should check the modelines you are using against the Myth database and see if they are optimum. The ATSC ones should work optimally since they are what comes in on broadcast.