I don't think it is the pin, it's made of tool steel and very tough.
-3 is very cold for a CO2 gun, yore lucky it even works at all.
My .22 on CO2 will 720fps at 85F and 520fps at 40F and 480fps
at freezing 32F (14.3gr CP)
So I think your speeds are pretty good.
You cannot get a new pin, you'll have to replace the whole valve.
But I really don't think it's the piercing pin.The 850 Store