As already been said mate depends how far you want to go. Also have to think if you want to sell the car on in future, and its got holes drilled all over then going to be harder to sell.

How far is your drive to work?

Plastic windows are a good idea but if you have a sliding front section then its easy for someone to break in. I like Dans idea of just doing the back window and quarters tbh. I would loose door cards and replace with a sheet of thin ally, bin standard seats and bolt passenger one directly to the floor.

If its got a sun roof i would remove that and just have a blanking pannel put in. You wouldnt belive how much weight is in a sun roof.

Unsprung weigh is where you will notice it more though. Alloy lower arms, Alloys bakes, and Light wheels.

As for everyone saying about loose some weight, they do have a point but its not the be all and end all. I am defo not a small chap and when i go, go-karting with mates who weigh 6stone less i still manage to show them a clean pair of heels.