1. You would have to be mad to do it these days unless someone basically gave you the upgrade, or you were able to buy it for next to nothing.
2. Yes, in general, you can gain a little in MacOS9 with some applications, and a good deal from the G3 -> G4 move in X. But —
3. No, it won't run like a Quicksilver. Ever. Putting a 12-cylinder Jaguar engine into a 1974 Pinto does not give you a roadster. All it gives you is a car that can't keep up with its engine....
In short, it is not worth spending money on. You can buy a nice dual G5 2.3GHz (the best of them all) for maybe $450, and it is even questionable (given the cost of a Mac Mini or an iMac) whether that would be money well spent.
In my view, a realistic plan for speeding up your PTPro would be to put 8.1 on it and run period applications.