Just to throw more fuel on the fire...
I can't see how removing the rear seats would have ANY effect with regards to insurance. As long as it was literally just the seats, and not the rear plastics/carpet etc etc. What's to say you haven't just popped down to the tip, or to buy something that wouldn't fit in otherwise, or...shock horror, to fit bikes in!
If the front seats had been replaced for bucket seats and harnesses, they may have a slightly different view however.
I COMPLETLY stripped the inside of my a3 (as in no plastics, no carpet, no door cards) and it made no difference to the insurers. Maybe that was because I was on a specialist policy, who knows...