Yes they can be polished but it would take you a longgggg time. Have you ever polished a wheel? Well you would have to polish all the way around all four pipes on both headers getting in between all the pipes.
Not something that I would really want to try. Yes it could be done but being mill finish your would first have to buff them with ruff polish then go down to fine polish. That's why we polish the pipes before we make them because the pipes are just straight lengths of pipe and we use a pipe buffing machine to do it. Then we do the cuts and bends and weld them up. After that we just have to do some touch ups on the polishing.

