What are they at the moment ?
Hard isnt necessarily good Stu - suspension travel helps keep the tyres on the ground. The lap times prove what is working and what isnt.
I would say 1 min 31 is working pretty good but with accurate geometry set up would be able to get that down.
Spend the money on springs if you want but if you have wheels pointing in different directions, caster angles all over the place, camber not set up and thrust angles that are causing problems then I would say it is money not very well spent
When the geometry is set up - using a tyre pyrometer you can measure tyres temps to find out whether the temps are even across the tyres and then take it from there. Being a tight wad myself, i would get the best from what is there before jumping to a diff set of problems.