I'm no expert, but I've been doing quite a bit of reading and research leading up to my own engine work. I believe it is pretty much accepted that the XS650 head does not respond well to the average port job. If you want guaranteed horsepower, my suggestion would be to go to the guys who've already figured out how to do it.
Purchase Craig Weeks' performance engine mod manual at
www.650performance.com. Note the title of the first chapter.
The Reality Of XS650 Cylinder Head Porting
Weeks works closely with Michael Morse at
www.650central.com. Through him, you can purchase the CNC'd head Weeks talks about.
Note well, I am no expert. I'm just pointing out what I've found from my own research. Personally, I'm not looking for massive HP gains, just getting the best out of what MamaYama gave me. To that end, I'm going to do only a mild cleanup of the factory casting. Keyword: mild.