Tiger is very fast on my G3, and there's no reason why it shouldn't be faster than Panther as it is more optimised code and the only new features that might drain performance, Dashboard and Spotlight, can be disabled if you like.

There is no serial number with OS X, so nothing to identify each copy even if it does talk to Apple (like in the OOBE sequence if you're online). I don't think it talks to Apple when you enter the iWork serial either, the serial is simply accepted by the setup wizard, and then it will only talk to Apple at the later point of running one of the apps for the first time and you choosing to register your details (presumably for spam, or so the Apple Police can come and find you if it sends the serial number back at that point and they find it's the same one that everyone else has) with them (you have to click "register later" about three times to get the option to "never register" - honestly, I can't imagine the outcry if MS were as invasive on privacy as this, sending your details to them without warning when setting up the OS, for instance!). Or at least I know it is possible to download the iWork '08 trial from Apple and then unlock it with any serial, regardless of how many times it's been used before (not from experience, you understand, I just, umm, heard this, umm, somewhere ).