Hi John
I've never sailed a Spitfire so wouldn't be able to discuss the different tiller weights. However, my rudders are racked as far forward as possible in their cassettes and the shrouds are either in the 3rd or 4th hole down. The distance between the bottom of the forestay and bridal shackle varies between 70-90mm. These settings normally produce near enough neutral helm both up and downwind. Can you remeber your current major settings?
As a side, when I first had 555 the tiller was heavier than 516's but adjusting the rudder angle/T foil pitch made quite a pleasant difference. I hope the attached photo shows what I mean when I mentioned the rudders being racked forward in their cassettes.