I'll add just a little here ..... I currently have a really nice OWB holster model BH 21 P custom molded to my PF9 by "Black Hills" leather and I love it. But then I'm a confirmed 'leather' man and don't really care much for Kydex and similar. Nor do I really care much for the IWB. However, back to the question ......
My Hellcat w/"SMSC" red dot will fit into tha PF9 holster, albeit just a tad snug, within just a smidgen of clearing the red dot sight on the front edge of the holster. If this was the only holster I owned, I wouldn't hesitate to use it with my Hellcat.
I have, since owning my Hellcat, gone back to "Black Hills" and ordered the same model holster for the Hellcat w/red dot sight. In talking with owner/maker about the leading edge of the sight vs top edge of the holster, he assures me the new holster will be custom fitted to the Hellcat with the red dot mounted. I'm taking that to mean there won't be any interference between the front of the sight and the top of the holster.
regards,
jumpinjoe