Unfortunately, the older X series were heavily based on production pistols, hence the P226 X5. Not at all saying this is bad, but it does limit you to what you can put on it. I assume this is also one of the reasons they have changed the X series to a modular design for those that wish to add optics to thier pistol without milling it. May want to ask the same question over at
www.sigforum.com, they may be able to give you a good direction to go to.