I have 17" GTA Teledials on my 147, no idea how heavy they are compared to other alloys.
Suspension and brakes are in the process of being improved.
You still didn't specify for road (always) or track (temp) but I will assume it is for road. Provided you understand sacrifice of taking out/putting in incase you need to use them.
Here are my tips;
1. Spare (on MY2005 this is spacesaver so weight is less than normal spare) - remove but understand sacrifice if you get flat.
2. Tools, manual & street directory, all ashtray/cupholder inserts, stuff in glove box, clear inside door pockets, floor mats.
3. Rear seat headrests, parcel shelf, underhood sound proofing, carpet in boot, minimal water in window washer.
4. And as for wheels, it goes like this tyre rubber is is lighter than aluminuim alloy so the sacrifice of lower profile is better cornering but at a penalty of weight. The advantage of alloys over steel wheels is weight but as it is unladen weight (on the ground and not being carried like the body) the greater advantage comes from quicker response from suspension reacting to bumps not from accelerating.
Now I am not suggesting I would do any of these but if you feel the need to lighten, go for it. Oh and if you have a sunroof...forget this because glass is heavier than steel roof(+ mechanism) and the weight saving I suggested would be neutralised