that amount of caster would aid in going in a straight line, but those sure are beefy looking for a drag arm.
although that does look excessive, throw it together and see how it rides. in theory, it will of course be more of a fight to turn and stuff, but it may not be that bad. i built a lot of caster into my drag frame arms and never really noticed a "difficult" turn. of course i wasn't going through the trails, but i didn't really notice anything weird.