On second thoughts I have to agree with choctalpur, leave it alone, well meaning changes without the right know how can make matters worse.
On a separate note, I believe that proper (appropriate) coils will not only improve the ride (metal AND offroad) but improve articulation and increase offroading safety with an edge over leafsprings but the expense is not worth the minor difference especially with the risk of not getting the right tension springs.
I would be interested in the firsthand experience of anyone who has used coilovers in a SJ410/SJ413? I know there are many out there as I saw several at the Sind Desert Rally. These guys I assume used the coils for performance rather than for comfort.