If you don't care what the door looks like, it is very easy to make the factory doors extremely lightweight. I did this on the 24 hours of lemons cougar, and while I never took the door off to weigh it, it can't me more than 10 pounds anymore. The heaviest part of the door is the intrusion beam, however removing that with the door intact is pretty much impossible. If you could get a replacement skin for the door, you could cut off the skin, cut out the intrusion beam, and put a new skin on, but I don't know of any source for a new factory door skin. Without that, you have to cut apart the interior shell of the door to get access to it, and there is no easy way to make that look good again.