I use both a 1-3 shimano raider and a 2-4kg dropshot for flathead. I actually prefer the dropshot for flathead. It's matched with a shimano symmetre 1000, 4 pound braid and 6pound fluorocarbon leader and I haven't had a problem bringing in a flathead to date. I've landed them up to the mid 50cm range and it's been a fun fight. There would be no harm in running a heavier leader as flathead aren't too finicky though I use the dropshot for bream also hence the lighter leader. Your setup sounds fine, if you're using jigheads you probably won't have any dramas with being bitten off anyway due to the longshank hook style.
As for casting weight, I normally use a 1/12th sized jighead and I find that with a school-sized prawn that's probably the heaviest I'd go. I don't know what it weighs all up but it casts well up until that point. I'd imagine that you could cast heavier (I personally don't like it) but just remember to be cautious as graphite rods will rarely forgive you if you overload and then try to whip it put a mile.