I noticed that your minimum radius at the reversing loop was O-54, but everywhere else on the layout was O-72, if I'm reading the diagram correctly. This reduction could significantly decrease the number of locomotives you could run should you want to have larger, scale locos (or large rolling stock) in the future. Probably not a big deal if you're only interested in the more traditional O gauge/O-27, but since you're using O-72 everywhere else you might as well keep your options open. I don't have your software, so I recreated your layout in SCARM using O-72 minimum radius curves.
I made the baseboard 8x16, as in your original text, but the diagram in your original post appears to be 9x15. I added an additional foot for a 9x16 layout, if desired. I used Ross switches (& curves, as they measured better w/the switches than the GarGraves) and GarGraves track & flex track. I switched the switch crossovers to a Ross 11 deg. double crossover piece for simplicity, and included a second one on the back of the layout for fun (but really just to help me ensure spacing between the lines). I put an optional "outer loop" siding on the expansion baseboard in place of the short sub-siding that you initially had in the lower right. This would be more functional, as you could put one or more industries and/or a passenger station here, or just park a train, while keeping both main lines open.
I agree with the suggestion of an "un-reversing" loop, though I'm not sure I know how to fit one in here, at least w/out making a second level, as another poster has suggested. For entertainment value, I've also attached the SCARM file for Phase 1 of the layout I'm working on, which can be seen in the "New Layout Idea - Thoughts? Suggestions?" thread in this forum. It's a 12' x 18' area, but could possibly be modified to serve 9x16. Good luck!