Wind the boost up and drive it hard for a few minutes, that will quieten it down - permanently

Less drastic, push it down the road then start it

Dunno about magnex, but mongoose (4" pipe, 3" tail) is louder at idle on my 4x4 than the scorpion (3" tail) on the 2wd.
It's more of a burble on the mongoose than the scorpio.

Give it a few revs and the scorpion is easily louder than the mongoose though.

As above, re-fit a centre box first, see how it goes, if still too loud, you'll have to try another exhaust.

Mind you, the original ford one was pretty quiet, but expect a slight loss in mid-range torque (I felt the increase over standard when I upgraded to mongoose on my stage 1 4x4).
The only problem is that it's not cheap and made of mild steel