I can't comment on having FSX/P3D on the same drive, due to when I made the transition I was able to add two SSD drives to my system.
I'm currently running P3D on one of them, along with my modeling, graphics, and other related programs.
I've read that FSX or P3D should be installed on the default drive for best performance. Having both of them, I didn't bother with for migrating FSX root locations to avoid redundancy of space needed. The way I see it is if a developer C's their product profitable to the consumer, they will provide a dual installer as needed so the installee has that option. When I did the transition I wiped FSX off my desk and kept going.
I know my response wasn't necessarily what you are wanting. Just a little input if you know what I mean. I think once you make the choice you won't regret it as a whole experience.
For the price of upscale add-on, P3D, I feel the transition was well worth it.