I don't think toxicity (LD 50) is relevant for plastic consumption. This sort of stuff packs into the gut and doesn't move or decompose. So the animal has to vomit it up or get surgery because it won't move down. I think the relevant comparison is ratio of pyloric sphincter between two species (the exit of the stomach). I'd be curious to know what that ratio is and it might be surprisingly close to one almost certainly less than 10. I do not know what the skill with which a whale can vomit, I do know that my lab mix pup is especially skillful.