I've never tried octave stringing on my OM or mandocello, mostly because I haven't seen any need to. One of the things I look for in CBOM instruments is a deep bass sound, so it would seem silly to change that by adding an octave. I also play mostly accompaniment for songs or tunes and I use a lot of arpeggios where I want individual notes to sound clearly. I couldn't do that with octave stringing.
That's just me though, and you might want to do different things with yours. I think this is really a case of finding the right tools for the job: if octave stringing suits your playing style and ears, then go for it.
On your other question, there are pros and cons to 5-course instruments. My 10-string mandocello is tuned CGDAE. It gives great range for arpeggios, where the low C gives a real depth to the chords. It's ok for gentle strumming, but the low C is a little too low to sound convincing on anything more aggressive.
For me, 5-course instruments really come into their own when fingerpicking. My guitar-playing fingers can cope much better with two courses for the thumb and three for the fingers. I can get an alternating bass that I miss on my OM.
Again, it's about the right tool for the job. If possible, try a few instruments of different types and see which one feels best under your fingers and makes the kind of sound that you want to make.
"Trust me, I speak with the utter confidence of someone who knows absolutely nothing about the subject."
David Oddy Cello-Mandolin
Fylde Touchstone Mandolin
Moon 10-string Bouzouki