I think weight certainly makes a difference to the 850's handling and every bit you can shift from the rear to the front is going to help. My coupe is around 600 kilos. Most of the weight saving is from removing the sound proofing and replacing the glass with plastic. Alloy front fuel tank and radiator etc. Fibreglass bonnet and engine covers and some metal sheeting removed. I did have a stainless steel muffler acoss the rear and when I changed to just a hot dog muffler alongside the engine the difference in handling was immediately noticeable. I have a lithium battery up the front that weighs about 2.5 kilos and it's going to be replaced by an agm battery of about 7.5 kilos and I am not worried by the extra weight at the front. The handling problem I think is mostly from the added performance and the far better tyres etc that we are using with our 50 year old cars.
The front is very light though.