Actually, I think Lexar does have some 8X cards available too...
Hardware is not the issue here; the card will get written in and read from as fast as the writer/reader (or camera/device) can handle.
Think of it as a hard drive; 4500RPM Vs. 5400RPM Vs. 7200RPM. A 4X CF card will definitely improve the file transfer (and read/write) performance because it is most likely the bottleneck in pictures taken for instance by a 3.3 MP digital camera at high quality settings. The buffer where the just-taken picture is temporarily stored is certainly very fast, and it can certainly take advantage of a faster CF card. Please correct me if I am wrong.