The pros and cons are all based on what was learned from the porting process. Unfortunately the confidential nature of the software prevents discussing it in greater detail.
Go can be a replacement for C++ for programs that didn't need to be written in C++. There aren't many programs like that around nowadays however, as most of the time C++ is only used when it's really necessary, such as for extremely latency-sensitive applications or applications requiring precise memory/allocation control.
I wouldn't bet on that. I would expect C++ programs to have been written ages ago and not having been rewritten because of a combination of "it works"/"I can't read it"/"It's not my code". The most famous C++ to Go port must be dl.google.com (http://talks.golang.org/2013/oscon-dl.slide#1), which arguably never needed to be written in C++ in the first place, except it was probably the only reasonable choice at the time.
btw, I think go is really not a replacement of c++. In my experience, go is more a replacement of java to improve the development speed.