I can't tell the difference in the two models performance wise. I have a sedan GSR and am quite happy with it. It really boils down to your choice of style IMO, some think the 2 doors are sportier looking, some think the Sedans look better. Its up to you. If you know you want to get a 4 door car, then a Sedan GSR would be a nice choice. The weight difference is somewhere to the tune of 100lbs or so, isnt really going to make much of an impact for street/daily driving.

As far as the JDM conversion. I dont have any personal experience with it yet, but have seen pics of sedens with JDM front ends, so I know its possible. As I understand it, all the front end parts are interchangable between the 2 and 4 door models. Someone correct me if I'm wrong.