if your Metal is 2011+ (134ps) then it already has a different map than the standard Mk7 Zetec S, so if you're looking at map gains quoted for standard ZS's then they're not equivilant.
The S1600 (2011+ only) and Metals are one-and-the-same so were sold as 'remapped' from the factory - I doubt this map is as tight as a pro would be able to get - I'd imagine it would be still quite 'erring on the side of caution' but IMO it's not worth it.
You already have the gains from everything the metal/S1600 has (shorter, faster, gears; improved intake, biger downpipes / exhaust, remap, dat gurggle), so from here on I think it you should be looking at sport cat, cams, induction (which are ell expensive) THEN a remap.
If you look at the Mounttune packages they will give you an idea of the correct upgrade path for the metal/s1600 and you will note that some of them include a remap because extensive work has been done.
I only had the standard filter on my 2011 S1600 for a day before putting the K&N on and there was a slight increace in induction noise but not as much as you would gain from replacing the whole thing with a J1 automotive indutcion kit for example. I'm still running mine with the K&N pannel and although I am toying with the idea of the J1, I'm quite happy with the KN pannel for now.