^^^^^^ This man knows what he is talking about.
Aye, our kit car only has about 100bhp but it'll outdrag pretty much anything on 4 wheels to 60mph. It suffers with the aerodynamics of a breezeblock but hell of a hoot off the line. And it's all down to weight.
You don't buy a 1500kg+ car and then worry about its weight, most people buy a Brera for the looks and style than the ultimate performance. By all means buy some wheels because you like the looks, don't buy them to save weight.
To save weight, take out the rear seats, aircon unit, airbags, stereo system, soundproof stuffing, spare wheel, and all the interior trim, carpets, all the electrical gubbins you can do without, replace panels with carbon, windows with perspex etc .... or for the money, just buy a weekend toy. Weight saving is much more than just a set of wheels