really, as was mentioned, the ioncannon is for SM and necron killing. at the last RTT, there were 3 necron players (granted one of them was me) but one of them even took a NB... and it was only a 1250 point game! basically, if my tau were to go up against that army, rest assured that this list could probably down the NB in 2 turns.
as for dropping 2 broadsides in favor of a 2nd hammerhead, in the end, i would prefer to have more railguns than a submunition shot. think of it this way, an ordnance shot will usually hit about 4-5 spaced out models. for -1 Str/AP over the submunition shot, i got 8 shots from the broadsides. if i focus the other broadside on it, then thats 12 shots, and since 2 of them have multi trackers, i can add 2 railgun shots. on average, this will kill more than the submunition.