ah good 'ol codex chaos marines and its 4, (yes, that's right 4!) seperate printings...
now here's the annoying bit - they fethed up the doom siren rules in the lastest printing!
the correct doom siren rules are in the 3rd printing of the codex... (2004 or so?)
IIRC, a doom siren doesn't negate the negative effects of the power fist. using a power fist mean your actual I vaule drops down to 1! so striking in I order you would strike with I1 due to the fist...
basically IMHO, slaanesh champions are the perfect candidates for the old power toy & fist combo; against generic troops you can use the power toy to strike first while against much thougher opponents you can switch to the fist...
cheers!