hi
most people just put a K&N panel filter inside the original airbox
the standard airbox is very good and has a cold front air feed, which is directly in front of the grill, giving some ram air type flow
would make next to no difference on a diesel
On the S-type R's you actually lose power by fitting a cone filter, on street/town driving, due to heat soak from the engine bay, the engines generate loads of heat in the engine bay at low speeds, it sucked up by the engine, due to high temperatures, the ecu retards timing to compensate, giving a loss in power
All high quality race type filters use either a panel filter or cone filter in a sealed box, with a cold air feed direct from the front of the car, standard air box offers this, especially if a better filter is fitted, like a K&N
if you want more power get it mapped, but make sure you change the oil regular and keep an eye the oil level does not go up on its own, due to leaky injectors, as this is the biggest cause of snapped cranks on this engine
also any air filter that offers more air flow or performance, can only be acheived by offering better flow, which can only be done by sacrificing filtration, so more dust and particles will enter the engine, causing unnecessary wear, fact.
cheers
Joe