From my one experience if working for an insurance company currently, it's more to do with the claims experience of a particular kind of vehicle......
Example to prove this would be 3 cars......
Honda Civic Type S GT 2.2 Diesel- Insurance Group 10 (old school system)
VW Golf GT Tdi 2.0 Diesel- Insurance Group 14
Audi A3 Tdi 2.0- Insurance group 11
All vehicles are 140bhp (as near to as your gonna get) 0-60 virtually all the same..... Yet the Golf and A3 are deemed more of a risk due to amount of claims, such as thefts accidents etc, insurance companies can only use statistics to calculate a premium.
So to answer the question relating to the Civic's a 3 door will be deemed in this case a higher theft risk opposed to a 5 door I supposed due to the sportier style.
Hope this helps
