640kg is next to near nothing! 714kg is still very good, can you offset any of the proposed weight gains (assume traction bar and bits or are you thinking of attacking the pies..) with some more weight loss i.e. drilled door inners, etc?
I was amazed at the difference throwing a few things out of the 356 made to my times e.g. spare wheel, heavy toolbox, etc. I was kicking myself at the end of the day though when I figured out how much weight in fuel I was carrying. The fuel gauge bounces all over the place so I kept brimming it everytime we had a stop. With a 55L tank I worked out I was carrying nearly 40kg in fuel alone, might have been able to shave another couple of hundredths of a second off if I was running on fumes
Imagine what the paint alone must weigh.......!!
Strip the fucker back to bare metal............ you know it makes sense........