While Agathis looks alot like mahogany it sounds a bit more like Alder. I used to have a DeAmond M75-T that was Agathis...I loved it, that guitar sounded amazing...
i'd say it's weight is way less than mahagony since i own a mahagony Beast(2004 Nj series) and could compare it to it's platinum counterpart which was agathis. For me it's definitely not as heavy as mahagony