I'm thinking of spending $10 to buy the Super Mario Run upgrade. If I link a Nintendo account to Super Mario Run, (I already did.) and if I buy the upgrade, will it sync to my Nintendo account and transfer over multiple devices?
Yes, it will. I purchased the upgrade on my iPad. When I ran Super Mario Run on my iPhone, after logging in to my Nintendo account, it not only synced the purchase but my progress (levels completed, coins, etc.) as well.
I don't know if it matters or not, but I use the same iTunes account and Nintendo account on both devices.