The article Optimal Chunk Size for Raid 1 and the zcav Benchmark has examined the question of chunk-size. Although dating from 2008, it has concluded that chunk-size makes no difference at all. In fact, he found out that the kernel actually completely ignores chunk-size.
I would conclude that chunk-size has no importance, and for RAID-1 disks performance considerations are the same as for ordinary disks, where performance is only dependent on sector- and block-sizes.
My logic says the same : I cannot imagine why a kernel would require two types of identical mechanisms - blocks and chunks. As far as I know, reads must be done in blocks, as the kernel cache is block-oriented (this explains the findings in the above article). Chunk-size could at most affect sector allocations on the disk, meaning that the allocation of files is done in chunk-sized units. Chunk-sizes for kernels that do that (if any exist) would then have an effect on disk fragmentation and file-sizes, but have marginal or no effect on read performance.
The only improvement in the performance of RAID 1 lies in its redundancy, when read requests are distributed between the two drives. But this has nothing to do with chunk-size.