All vehicles are going to 'scream' downhill... they will reach the highest RPM they will ever get to... 'screaming' is just a description of the sound... not something to worry about really.
The carb on the PA-50I doesn't act like its supposed to.. and I am still figuring out how to work with the little piece of junk.
Pipes will make more top end RPM and speed... and as long as you richen up the jetting you should be fine.
(but any engine that is run harder is more likely to have problems anyway).