The bolt on filters do nothing apart from make the engine sound louder, in fact you'll probably end up loose performance as all it will do is suck warm air in from the engine.
Exhaust wise, you can either fit a full GTI system from the manifold back, or get a downpipe from a 1.8 driver which fits the 1.6 manifold but is thicker so you can fit a GTI system from the centre box back.