It's really quite simple. Take some 0000 grade steel wool and soak it with glass cleaner... You get results like this every single time. So simple yet so cheap.
If you want to polish them out you can use stainless steel compound (green stick) with a 3" buffing wheel on a drill of grinder. Just remember the smoother and higher gloss it is the more it shows imperfection or scratches, thus the more it is to keep it at that point.
and yes I am crazy en off to polish the cross member
Since they're new, if you just want to clean them and bring out the shine, you could try S100 Polishing Soap. It contains very fine clay that also removes discoloration and very fine scratches, and leaves behind some protection, as well.
I stored my car a whole winter once and forgot to clean my exhaust tips. I first clean the soot with wheel cleaner. I finish the rest with Autosol metal polish. Worked really good.
Usually I wash them at the same time as my car every week in the summer and regulars car wash soap does the trick as the Autosol leaves a protective film for a while.