BRS:
A spacer behind the spring will only reduce it a bit, but is easy to try (I use #10 stainless split washer). The Model P and Cattleman mainsprings should be the same part (you can verify this by parts lists on vtigunparts.com and cimarron-firearms.com . They do seem to vary, though. Seems to me that when Pietta started putting lighter mainsprings in their guns, Uberti lightened up theirs somewhat, but some feel heavier out of the box than others. I don't have experience with the Lee spring but know many use it. VTI carries the lighter Pietta spring as well. The Wolff spring and the ER are both a bit heavier than some light springs. You can always put the spacer under a lighter spring as well. I'm not sure if they still do it but the ER's used to be shipped with a spacer under the ER spring, so that in case someone ran into hard primer / light strike issues, they could remove the spacer for a bit more power.
edit: if you do order a spring from Cimarron, get the ER. The regular mainsprings have been in stock for a long time and are probably no lighter than what is in your guns.