You mentioned this is primarily a read heavy box, correct? If that's the case then going to RAID 5 might be the move to make if cost is a concern but keep in mind changing the disk subsystem is not something that should be taken lightly. Although writes aren't the majority here understand that they will suffer going from RAID 1 to RAID 5 given the penalty by design.

For every one write, you have 4 operations to do:

A) Read the data to be changed

B) Read the corresponding parity

C) Write the change

D) Write the updated parity

A lot of good advice already shared but I wanted to chime in with this factor considering you're likely not expanding space because you want to but rather you want to have capacity for more data to be written to those disks:-)