1st to 5th ratios are pretth much the same, then you have and added 6th for motorway use, if you were to do it you would need a new clutch and flywheel to, not tio mention the box is alot heavier than an F20 to
you get a box thats too big, designed for a tx box, irritating to set up the linkage and is pointless as the xe is mapped to be economic at motorway speeds, put another cog in there and you are cruising at 2500 ish and have less power to pull away from stuff whilst being lazy and not changing gear.
of course if you are a poser and want to say you have a 6 speed box then fine
if you want 6 gears go for a quiafe dog straight cut box and do it properly (oooh for the money! lmao)