Many people would say your assumption is flawed. Woofers are made of a variety of materials as well (doped paper, polypropylene, Kevlar, metal composites, etc.) in an attempt to keep the driver as non-deforming as possible. This is especially important in two-way designs because the woofers handle higher frequency ranges than they do in multiple-way designs.

It certainly may be true that better tweeters would improve your listening, especially if your current speakers are treble-shy and you prefer a brighter sound. But I would disregard the materials from which the components are made and just go with the sound. There are so many variables which can affect the sound of a speaker regardless of the materials used for drivers -- crossover design, placement in the room, the amplifier driving them -- that zeroing in on one type of tweeter is misguided, IMHO.

Thanks for the input, Steve, you made many good points about speaker components...