Well, I reduced my 0-100 gtech time from 7.5 to 7.2 on weight saving alone. With a number of other small mods, my car now runs under 7 sec, around 6.8 - without the weight saving, I'd still be around in the low 7s.
Complete tow bar assembly = 25kg
ALL sound deadning and floor mats = 25kg
Spare and jack = 15kg
Removed front reo bar = 5kg
75kg already. That's the same as having a passenger - and you NOTICE when you have a passenger in the car. It's a LOT slower.
and then if you want to get serious and remove the air-con compressor, condensor and accumulator, there's another 15kg.
replace the bonnet and boot with carbon-fibre numbers. 15kg
replace the front seats with racing seats. 15kg
etc