The springs used for the trunk are torsion springs and work by the wire rod being twisted. To get more spring, the rod simply needs to be twisted more. That is why a spacer would be needed to twist the end enough to get it to raise the trunk lid completely on its own.

Spacers would definitely help... but I'm afraid that to get enough torque to fully lift that heavy lid, you'd put too much strain on the metal (sides) and plastic (center) supports. Might be fine for a while, but it could rip out over time.