In terms of brake pad material, the term 'soft' is rather a misnomer. It really refers to the pads tempurature range and friction level. Hard pads = higher temperature range; soft equals lower.
I would doubt seriously that changing the pads will have any impact whatsoever on the effectiveness of the handbrake.
I am not familiar with the rear brake setup on the ARP4 but when I converted to rear disc brakes on my Plus Eight in 1990, I modified the handbrake 'tree' to increase the leverage and effectiveness. It rise to a state of 'just about legal'.
