An IK may add a fraction of power to a 16v/Willy on the rollers - with your bonnet open and cool air being fanned on - but in the real world youll lose a bit of torque and power in circumstances where cold air is not forthcoming (e.g. around town, at low speeds, in warm weather).
Id say if you use your car purely out of town and at fairly high speeds, then the much-known-of disadvantages of an induction kit on a 16v/Willy wont really effect you. Ive got one on my car, but thats because my car came with one and I cant seem to find an airbox for reasonable money. But Id think really hard about spending £70-150 on an IK if I was commuting or driving around town a lot. Mind you, they do sound soooo good.
An airbox is designed to provide consistency in the engines performance - if theres too much cold air, itll add some hot air from the engine; if theres too much hot air, itll allow some cooler air in. Id stick with an airbox in your case and just get a performance element. Theres a good reason why most race cars use them.