Sad thing is, I've seen some of the FD body kits where the replica had a tighter fit than the real thing. Some replica makers actually are really good at their craft and do a replica off the real thing, which loses even less. A friend of mine has had some of the best fitting replicas I've ever seen, to the point where you can't even tell it was a replica unless you knew what to look for.
However, there are a few things you need to pay attention to when inquiring about replicas, depending on the material. Some of the Polyurethane replicas don't use high temperature resin, as it's quite expensive, so your bumper can warp over time. Fiberglass, as was mentions there's stronger and weaker fiberglasses out there, it all depends on what's used.
However, as much as some people may balk at this statement: Signal auto in southern california actually makes some damn good fitting replicas. Also check out some companies such as BRS-Autodesign, AeroDuo, etc. Some good stuff can come out of those companies that I would buy their replicas over the real thing, as it's half the price and around 90% - 110% of the original.
To summarize: Knock offs can be good if you buy them from the right place
