Well As you can see I have the raised center in my scene but only have things in the distance from where the camera would see them if it wasn't taking in the entire panarama. I also need to level it out just a tad it appears. I originally rendered that at 10,000x5000 like you and it was a serious chore that took 24 hours to finish. When next I try, I'll go for the lower resolutions as well.