Generally speaking, I think you'd want to have the slide milled for whatever Reflex sight you want to use. This allows a lower mount and one that is more practical to get co-witness from taller open sights.
I understand SiG uses a proprietary mount pattern for the Romeo. For that reason alone, I would eschew the Romeo in favor of one of the several other good choices out there.
I read on another forum, an employee of Optics Planet posted that Holosun makes the rebranded Romeo for SIG.