Is Postfix a better MTA than Sendmail?
Hard to say. Which is better, vi or emacs? :) Since I don't know Postfix, the answer for me is "probably not"... YMMV however.
Would changing the Raq over to use Postfix have any advantages, if so, how easy would it be to change them over?
I don't know about advantages, but I know it would not be too simple... unless you didn't want to use the UI to administer any domains or users... and then what's the point of having a RaQ? There is so much that's hardcoded it would take more effort than it's probably worth to make the change. There's no abstraction layer between the UI and the underlying services like mail and web to enable easy replacing of one program with another... an abstraction layer like that would add a significant amount of overhead and speed decreases to accomplish every task.
Also, not so much of an issue now that EOL has been announced, but if you modified the Perl code, there's nothing to say that the next update from Sun wouldn't just overwrite your changes.