from what ive heard from my felow anglers is that you will definitely need 15lb + line i would personally look into getting 25lb+ braid if i were you, also you will need at least +- 700m of line on you reel depending as you need to fish out more than 300m....
like the guys here said you need the right stuff bud otherwise you end up filling the dam with gut, and you are bound to break your tackle if you dont go all out. also the storms are apparently very hectic so you need to look into something to help cope with that properly tie down your stuff etc. like SPINEDOK mentioned get a decent main line and also a proper leader to help cope with snags.... als you need to look at the terminal tackle you need proper hooks, proper sinkers, lead clips etc. decent lead core and 25+lb hooklink material.
other than that just make sure your line capacity is right with 8lb mono you not going to get anywhere and i promise you those fish will break that 8lb mono like nothing.... like the guys mentioned 25lb hook link gets broken by those fish.... go and look for some decent baid nothing less than 25lb if you can afford it. otherwise mono but mono at 15lb is at 0.3mm so you wont be able to go very far with those shimano 6000's maybe 300m of mono that means you can fish out 200-240m MAX..... so good luck bud.
my personal suggestion would be save up some money and buy yourself some decent big pit reels and fill those bad boys up with braid 25lb +, then you wil be ready for a dam like doorndraai.