where does your +50% come from ? There is soo many number here that it's not very clear, from the op number, 343% => 438%, this is "only" a 28% more. Well that not "bad" but that not 50% and that is not considering very late game technology of productivity on steel, plastic and low density structure casting that can all go to +300%... this would change all numbers and one would need to tell me clear number to start a debate on fact.
And then we could also argue that dev didn't intent low density structure casting to be "better" than the assembler one... I mean for me it's a recipe made for vulcanus. (it's cost vastly less plastic for example, and plastic is a potentially bottleneck resource on vulcanus)
I doubt it's a problem and I again think "vastly" is a big exaggeration.