Okay, then that generates a different question (one that also came up recently). If I have a squad of genestealers deployed in a line in a wood, and the shape of the woods means that some genestealers are 5" deep and others are 7" deep, and a squad of terminators that includes a heavy flamer targets them, can he kill the ones that are 7" deep? Those were the only ones in range of the heavy flamer, but he couldn't see them.
I guess the reworded question is this: does establishing LOS from one squad to another determine the casualties that can be taken? In the wreck example, the template reached to a large percentage of the squad, but only three were in LOS. In the forest example, the template reached to three genestealers who were not visible (and it could not have reached to any of the visible genestealers.)
If the answer is yes, then you place the template as you like, and then take as many models as casualties as you like, subject to the standard LOS and range rules and the flamer is reduced to a 6" weapon. This seems odd, given that a flamer "ignores cover saves" and by implication intervening terrain.
If the answer is no, then flamers gain a significant amount of power as weapons that ignore all terrain, and can kill any models within their 8" range, subject to the rule of not touching friendly models with the template.
Neither of those options seems satisfactory to me.