If you go with a ported o2 housing and big downpipe (ie 3") you may get some boost creep issues. Has the wastegate passage been ported on the 16G as well? If it wasnt, might as well do it while you have the turbo off the car already.
Even though a downpipe upgrade will give you quite some gains over the stock unit, I'd be careful about going too big, too fast.
I'd still port the manifold and o2, then see if youre getting any creep. If theres none, do the downpipe.
