While I cannot comment on how much it affects the pistol finish, I will say that I cannot recommend the Serpa as a concealed carry or open carry holster. It is too bulky, and sticks out too far for most people to conceal under anything but a heavy jacket, sweatshirt, or coat.
In addition, you run the risk of the mechanism getting clogged with dirt and debris, which renders you unable to draw your weapon. I have personally witnessed a Serpa get jammed with a small rock during a training class, and if you search a couple of gun forums, you'll read similar stories. Many handgun training schools, such as Gray Guns (IIRC), no longer allow the Serpa to be used at their schools.
A bunch of the guys I know, myself included, jumped on the Serpa bandwagon for "soft uniform" use when it was first introduced several years ago. As a result of further experiences and research, most of us have ditched the Serpa and gone back to leather OWBs.
IMO, the Serpa should only be used as a casual use range holster.
If you want to try OWB carry, invest in one of the traditional style (aka "pancake") OWB holsters. Every holster manufacturer produces some variation of this classic design. They're leather instead of plastic, can be found for about the same price as a Serpa (~$40), and work much better at keeping the gun tucked in close to your body.
Holsters by RGrizzle
http://lobogunleather.com/mode...ancake_style_holster
etc.
If you need the retention of a Serpa, get a thumb break on the pancake. That way, there is no mechanical component to fail, jamming the gun in the holster.
If you want the quick on and off of the Serpa paddle, get a pancake with belt snaps.
K&D Holsters | Custom Gun Leather and Shooting Accessories
http://lobogunleather.com/mode...loop_belt_attachment
etc.