IMHO:
Any infantry hit by a lascannon would be fried pretty much no question of that. However if you have 3-7 infantry per stand one infantryman getting fried won't do much to the stand as a whole. Plus at long range you'd have a bugger of a time hitting any sort of infantry. Infantry can spread out, and AP weapons can fire a barrage of shots in the enemy's direction (either fire rate or blast as in missile launchers).
Tyranids are an issue and here's my opinion on armour. Does the unit have vehicle grade armour and the size to go with it? If so is it completely covered by this armour, or just partially? If a unit had tough armour or carapace, but had either an open top or an area with lesser armour than LV would work. If the unit is too small to be compared with vehicles then reinforced armour as in terminators is probably best.
If battle suits have consistent armour levels then I would guess good armour as infantry, possibly with reinforced armour, would be most appropriate. The light vehicle classification to my mind represents inconsistent armour protection.
I find the idea of -1 to hit LVs with AP interesting though. Represent bullets pinging off the side of an assault bike?
