I obviously use the 8.25mm myself and I too see the slight distortion around the edges and it is my thought that the distortion is from the 8.25mm lens itself. Because there are so few replacement lens manufacturers, the choices are limited.
Peau Productions 8.25mm f/3.0 41d HFOV 16MP BackBone System H7PRO – MODIFIED HERO7 BLACK (Does not come with the proper lens, only stock lens)
Eagle Vision Customised GOPRO 5-6-7 lens 12MP M12 12MM In my opinion, the Backbone option is really designed for the guy that wants to use larger lenses. In order to use the Hero sized lenses, you screw an adaptor on the BackBone which essentially puts you right back where you started by threading in a GoPro compatible lens which you can do without the backbone conversion.
The guys at Eaglevision actually had theirs made because they were not able to find what they wanted on the market.
This post is not intended to be the end-all as it relates to lens availability, it is my addition to the collective from my experiences and may not necessarily capture what is currently available on the market.
** SPOILER ALERT ** I recently received the new Eaglevision lens and will be sharing a video soon of its performance along with all of the other current scope cam options
Listed Alphabetically Eaglevision Cam GOPRO 5,6,7 Side-Cam kit
Hajimoto's Scope cam assembly (only for comparision)
Phone Skope (Gopro Hero 5,6 or 7 version)
RusBears Scope Cam (EKEN camera)
SideShot For Gopro Hero 5,6 or 7
TactaCam FTS 5.0 System (Proprietary camera)
As you can imagine this will be a large video production and will take some time to pull together but its in the works
Hajimoto