I fished Llyn Brenig when I Captianed the American Team at the youth world championships back in 98. that is an impressive fishery. You are lucky guy to be fishing there. If you are bank fishing I can also understand your interest in shooting heads.
First of all if you are in a casting club that is your best shot. If you're already topping the 50 meter mark that would insure your position as one of the top American casters. I compete here in the states quite often. However, my greatest experience is with a 5 weight in competition. I did fairly well in competition this year.
As you know shooting heads are more of a function of a smooth slow back cast and getting your overhang to match your distance for turn over. The best tip I can give you is to delay turning over your hand. It really helps load the rod like crazy. It took my 5 weight distance to a new level. Another tip is to step forward into your final delivery. If you're over 50 meters I'm sure you are already doing both of those. I'll be glad to pass along anything I know.