I have a 147 1.6 and have done and noticed the following (pictures available on my site):
1. Replaced pedals with Sparco pedals (silver with black anti-slip rubber)
I personally find that after having installed the Sparco pedals, they "feel" lighter and I seem to have a better "feeling" of the pedals and the pressure.
2. Replaced gear knob with Momo all-aluminium model.
The Momo gear knob feels much more comfortable and is easier to use for quick shifting (when you don't really have enough time to actually get a proper grip of the gear knob. - I just push it towards the required gear). It also looks nicer with the alloy-look central console (A/C and Stereo).
3. I have used Foliatec interior window tint (35% filter effect) on all windows except the front windscreen (illegal).
Nothing to do with performance, but I like the way that there is a uniformity of colours on the outside of the car: Black and Red. I find that this also helps conceal the B-pillars. The car actually looks quite "stealth" when driving at night as there is very little outside people can see...
1. Replaced the original alloys with 17" OZ Super Novas.
Changing alloy sizes need a bit of thought as the wrong choice can completely undermine a car's performance and looks. I chose to go one size bigger (from 16" Alfa Romeo alloys to 17" OZ Super Nova alloys) and put some good tyres.
The result of putting slightly bigger and lighter alloys is that I have gained slightly as far as the top speed in concerned (With the current set-up, I have done and checked that the car reaches a maximum of 216km/h on a flat motorway with one passenger in the car with me).
1.a. Following the replacement of the alloys, I originally chose Goodyear's Eagle GSD F1 tyres (215/45ZR17) but I have now changed to Pirelli's SP9000 (215/45ZR17) which seem to provide much better grip and control.
I took the car onto a track day with the previous set of tyres (Goodyears) and found that although the grip was decent, they suffered a lot during a days "hard" driving. Since I have replaced them with the Pirellis, I have driven "hard" both in the wet and on hot dry surfaces and the tyre treads seem to still be in a very good condition. The actual "response" that I get from the tyres (particularly the front) is excellent, a real feeling of control.
2. Replaced original back box and exhaust pipe with a Remus exhaust system.
On its own, and after being "warmed up" the noise is slightly deeper sounding that the original back box and pipe. Combined with the filter modification, there is a slight increase in mid range acceleration (throttle response) and the noise has only improved (especially when accelerating between 3500rpm and 5000rpm).
3. Removed original air filter and tubing and attached a K&N sports cone air filter with a shorter tube.
On its own I don't think it would do anything substantially noticeable, but I did this conversion after the exhaust and I can certainly notice a difference (as previously stated above).
4. Replaced original springs with Rotec springs resulting in a lowering of 4.5cm.
Having changed the springs (not the suspension) of the car and thus lowering the car by 4.5cm, I can definitely say and witness that the car's handling has improved enormously. I have found myself going into and out of tight corners at a considerably higher speed than before having the springs replaced. There is a noticeable reduction in the car's roll and pitch, however you now feel even the smallest of bumps and holes in the road (probably due to me not replacing the suspension yet).
Anyways, I hope tha above is of some help...