The funny thing is, by setting it to allow apps to use cellular data, my cellular data usage did not go up, but my wifi connections became completely stable. Before, if I had say 7 apps that needed updating, it would update three or four, and then stop. And I would see things like apps saying "unable to get data right now, are you connected to the Internet" even though it clearly showed 4 bars of wifi up in the status bar. All those issues are gone, but my cellular data usage is just a few MBs.
Your slow OneDrive syncs may be something similar, so that might be worth a try. Change the setting, recycle the phone in and out of airplane mode to reset the wifi, and see if things start moving, and if so, see if your cell data usage goes up. If it does, change the setting back, no harm, no foul. It seems to me on the phone that OneDrive is auto-upload only, and only for photos, so it shouldn't go nuts and chew up your data plan, I think. Also, most of the high-volume system stuff will only happen over wifi (map updates, system updates, etc.). That won't change, but it again does seem like the wifi connection works a lot better if you allow cellular data too.
Anyway, good luck - hope it gets sorted for you, as the phone hardware is fantastic.
:-)