cone filter wont do nothing on a dci mate. your better off ordering a aftermarket panel filter to go into the original air box.
if you want, you could mod the exhaust system, ie de-catt and full straight through. it will make it more of a loud tractor noise tho.
few bhp unlocked from that aswell.
tdi tunning boxes i personally do'not trust. but each to there own.
saying that a remap would render the need of a tunning box usless.
as for a cost of a full remap, your best off looking around. RS tunning on here might be able to help with remaping your car.
clean service aswell on these goes a LONG way to keeping the brillaint MPG we all bang on about.
AND if your good at mechanical stuff, take the egr valve off and give that a dran good clean !