Lets get some facts and figures in this thread:
Stock seats weigh 58 KG for the pair, I believe excluding the electric motors + assembly to move them back and forwards. I can't check now as I'm banned from fchat again, they truly lost the plot over there. Depending what type of seats you go for, you can save as much as 46 KG.
Battery weighs around 18KG, a large lithium battery weighs 3kg. So 15 KG saved there.
Stock exhaust cans weigh an impressive 19kg EACH (!). Replacing them with a set of Fabio's valved items you'll save 24.2 kg, or the unvalved versions you can save as much as 30.6! And that's just the cans...I hope
@Fiammenghi Italia can advise how much can be saved over the rest of the system, I suspect quite a bit with the 6-1 exhaust! Lets put a very conservative figure of 60kg savings for the exhaust.
We've now saved 121kg without even trying.
There's quite a lot of weight to be saved in wheels (important weight too, as it's unsprung), but I'd be careful on that as not many wheels suit the Maranello well....
120KG heavier than a dirt bike. That's a lot of weight to be removed. Full set of Fabio's exhaust will unleash decent extra power as well, I believe close to 550bhp. Modern tyres, brake pads, better brake fluid...will be cheap fixes that makes the car feel better without negatively impacting anything. Power to weight ratio with this will be about on par with a 599 GTB.