Hi Cartman, I bought mine from Jones Springs, an excellent service as they machine them down to fit the 80 rears and retemper them.
The first answer is that mos folk are replacing stuffed standard springs with paras, but replacing with new standards will also give dramatic improvement.
The paras offer a great deal more flexibility, so give better axle articulation, again they do offer on average an inch or more extra ride height. But beware on the rear as this is where the problems can occur with both your shocks AND your propshaft as they can damage the standard shocks and if the axle flexes down too far cause the propshaft to come apart (especially on the 80") to this effect, I replaced the missing axle check starps to keep the axle in check. The fornt axles dont suffer as much as the springs are shorter ergo the the effects are not as extreme
I posted my experiences of upgrding mine in my thread here
http://www.landyzone.co.uk/lz/f6/killajoules-series-80-upgrade-project-224357-5.html
They certainly radically improved the ride on my motor as the existing rears were virtually flat with little room between the spring and bump stops causing the 80 to crash over the slightest road imperfection, now she sails along in relative comfort.
The only other thing to point out is that the springs are half the thickness at the centres so the U bolts extend significantly through the bottom shock mount places. I cut mine down which of course melted the nylon in the lock nuts abit so I am constantly having to retighten the front U-bolts.
However as a regular trialler I can honestly say that the supple suspension dramatically improves the traction.
There were some dodgy early parabolic springs around that were notorious for failing but as I say my experience of Jones has been excellent and of course Rocky Mountain have an excellent reputation.
I have on the flipside heard from fellow trialers who have returned to standard springs as they were not happy with the paras being too soft. So depending on what you put the series through just replacing with standards may offer just as good iprovement in performance.