eudaimon:
I think it is quite simple, that really everyone agrees, and that there is just some misunderstanding of what people are saying.
I think everyone agrees that you only hit what is under the template AFTER you have rolled the “overshoot” part. The only thing people are confusing is your own personal play style to establish that.
Personally I put the flame template down on a ranked unit so that it covers the first rank and just a fraction of the second. I declare that I am aiming 2" short of the back of where the template currently is (in other words, if I roll a 2 on the dice, the template stays in place). Then I roll the dice and move the template forward (or not, if I roll a 2). So if I roll a 6, I move the template forward 4". Then, and only then, do I count the models covered by the template.
Anyway, back on topic, I see magma cannons as almost essential in the list. We have 3 tools to deal with monstrous infantry and cav effectively. The first is magic - mostly Flames of Azgorh. I once deleted an entire Gutstar (left some characters) on turn 1 with an irresistible cast and no scatter. Nice (nb. ofc against ogres Purple Sun will do a similar job, but it is less effective against higher I armies). The second is the K’daii, but fighting monstrous stuff can be a bad target for the K’daii, as you don’t get thunder stomp and your flaming hits will generally not do much damage, also they tend to have higher strength multiple attacks to whittle down the wounds. That is where the magma cannon comes in. Str 5, armour piercing, d3 wounds. Its almost purpose built to kill monstrous stuff.