(edited - see below for updates)
I think you are right about the 2013 spring being stiffer at the 6.9 N/mm rating than the previous stock springs for 55 coil forks. Also right about the new ones for 170mm forks only coming in the one "stock" rating. I'm guessing there is only so much you can do on the damping side to overcome a spring that is too stiff for your weight. I got slightly better results switching to 2.5 wt. oil. I'm only guessing here, but maybe Marz decided with their limited 2013 production to "average" the spring rate and only make one spring for the 55 RC3 V2. Whatever the reason, I'm not happy they don't have spring options. If they were going to only concentrate on a few models then they should have done a thorough job on those limited fork offerings. I doubt you and I are the only ones Marz is hearing from. Hopefully a lighter spring will be forthcoming.cancel my thread above - apparently there is no other titanium spring available for the 170 mm fork, according to Marzocchi.
Anyone know anything about changing the shim stacks - how it might alter performance, how to do this?