There are far too many factors to give a generalized statement on that question. If you're doing mostly large monolithic file transfers, adding more drives could help because you're spreading the load over multiple drives. The parity calculation shouldn't even be noticeable in a healthy array. Are you using software raid or is it done by a raid card? If it's done by a raid card, I wouldn't even worry about going from 6 to 7 drives with regards to parity overhead. I'm running two sets of 10 drives in software raid 6 and it's not even a concern.What is the theory of increasing number of drives vs. % of capacity on max. RAD performance?
If I have six drives in RAID 6 and add a seventh, will that cause more CPU load from the parity calculations?
At this point it is about 60% full and would then be about 50% full, but usage will increase in the future.