I have tested this a couple times, and made sure that the miner was attacking a 90k+ mineral asteroid each time, in order to remove variance in travelling from asteroid to asteroid.
When out of sector (OOS), the ship was making about 600-700 iron per 5 minutes.
When in the sector, the ship made 2000 iron in less than 20 seconds.
Another thing to note, it seemed that when OOS my income of iron was only ticking up by 1-20 bits at a time. This is interesting, because it means that the miner was never receiving the "big chunks" of minerals that come with breaking a section of asteroid.
With the OOS mining being this slow, there must be something wrong. Sitting in the sector and mining for 5 minutes is currently equivalent to leaving your miner in a sector while you are OOS for 5+ hours.