In my experience, the post sizing ring does deform or otherwise destroy my cast boolit (i.e. size) and I would not personally recommend them to anyone. I suppose it actually would depend on what size your cast boolits are, and what size the Lee FCD post sizing ring is, in the inside diameter.
Yea the FCD's seem to be hit or miss. I like using them and they work great for the purpose I need. If I got a FCD that was not right I would get a hold of Lee and have them exchange it for one that is. I don't know maybe it's just me but I see Dillon people getting bad or broken parts replaced every day and talking about how great Dillon CS is. Then people that get a bad Lee part don't give them a chance to make it right and tell everybody not to use them. I'm not singling anybody out here. It's seems to be what I see at many different forums.