Another thing that hasn't been brought up - are you facing superheavy vehicles or not? They ignore all penetration results except for Explode!, and those only remove a further D3 hull points rather than blow the entire thing up.
Against non-superheavies, attacking with AP1 weapons (Fusion, Railguns) usually makes a big difference since for every Penetrating hit you get 1/3 chance to kill the vehicle outright. Against superheavies, AP1/2 doesn't make that big of a deal, and you might have better results with massed fire. It's why Y'vahra is so good despite its AP3. With maximum luck you can get 6 S8 hits, plus 6 haywires.
If you're up against a superheavy with poor rear armor, e.g. a Baneblade with 9HP and rear AV12, Fusion is not that much better than CIBs. A single Fusion hit takes (prob of rolling exactly 4)+(prob of rolling 5+)*(1+1/3*D3) hull points, which is 0.0833+(0.8333)*(1+1/3*2) =~ 1.472 in expectation. A single CIB hit takes 1/3 hull points, and you shoot three of them giving 1 hull point. So CIB hits are worse (obviously) against the Baneblade, but not by that much. And they have better range and are better against the rest of the army.