You are probably right to assume that 501 is used in the main frame only, not that there's necessarily anything 'bad' about that. While it may weigh a bit more, beyond that, well, there seems to be a never ending debate about the 'feel' of various tubing materials. As long as you are happy with the bike that's the most important factor.
Only experience I have with 'cold setting' is bending mech hangers back into alignment. You'll need plenty of leverage and some try and error 'till you a get a feel for it. Certainly worth having a go, I reckon. Good luck!