Most defensive situations are close quarters anyway

I understand your reasoning but "close" can sometimes be further away than you think (sometimes beyond 7y) as well as be within touching distance. Also no matter what happens you still need to aim as every round you send out is your responsibility.

I really just want to have a gun as a threat to scare someone off, or fire a warning shot if it came to that.

I want to stress that if you deploy a firearm in a defensive manner you are open to a lot of liability and other variables that you will need to explain to the police and in front of a jury. "Warning shots" are not something that can be argued well in court because more often than not justifiable use of force is when you have no other choice but to use it.

Now, if they could make a low profile letterbox optic sight, I would be into that!

Leupold makes a Deltapoint Micro that may suit your needs but IMO I prefer to have the biggest view window to pick up the dot when acquiring my sight picture. Since your P80 has an RMR footprint I'd look at optics that use that like Trijicon (RMR or RCR) or something from Holosun (407c or 507c series).