I can't speak for 100-series owners in general, but I don't think there are many downsides to running 275/70R18 on the highway. You will have a slight pessimistic error in your speedometer, possible negative effect on fuel economy, and you might be able to feel a very slight hit on acceleration.
There are a multitude of threads discussing tire size if you'd like to know more.
There are a multitude of threads discussing tire size if you'd like to know more.