I think that the extinguisher positioning is functional providing the boot does not jam shut.
The quick release clamp on the extinguisher will provide a quick release.
I have my extinguisher on the passenger side floor, however, as being a Roadster I have no option unless I fix to the floor of the boot.
In case of fire, the first priority is to get the belts off and out the car, so retrieving the extinguisher from the rear strut brace should not prove difficult.
Regarding the look of the car, absolutely fine for track use, but ott for road use.
All you need is some extra power to match the looks.
Keep up the good work.