Most kits will lose you power as they are open
Hot air (less air) = less combustion = less power
You will need a decent enclosed one made of heat resistant materials (carbon fibre) and a well placed pipe to get lots of fresh air.
Something like a gruppe m, but its very expensive and will give you about 6bhp top end!
In short, no they are not worth it. Manufacturers spend thousands on things like air filters. If you are upgrading the filter its worth adding an exhaust and remap to fully utilize it and make a higher accumulative BHP figure (a filter could give that extra bhp to knock you up from 245 to 250 bhp for bragging rights)
