Talk to Frogs Island about it. They sell both OME and TerraFirma, so can give you the lowdown on both.
I bought my TerraFirma springs from them, but haven't finished the build yet to try them out :roll:
The new dampers with extra reservoir look excellent value if they are good quality. But they will only really come into their own if you are racing. If you are only doing trials or challenges, the dampers are not likely to get hot enough to warrant the extra fluid capacity.