Would I be correct assuming that this shirt is all but identical to the merino composite, minus the merino backer and a different neck?

Yes, that is pretty much the case. The Stoic Breathe Composite and the Stoic Merino Composite ¼ Zip shirts have the same tailoring, the same outer face, and the same fabric at the neck. There are only two differences besides obvious ones like a zip neck vs. crew neck. The Merino top has a fuzzy merino inner face (good for odor-resistance and insulation when wet), whereas the Breathe top has a fuzzy polyester inner face that is slightly thicker-knit, giving it a tad more wind-resistance than the Merino version offers. Whether your personal preference is synthetic or wooly, both tops offer great expedition-grade insulation and wicking.