So 264MB/s seems to be around the max interface speed for the setup you have connected the LEs now, since 0x00 should be extremely compressable. If the drives had come with SATA 6Gbps, it probably would also max that interface for writing 0x00.
Can anyone here with more programming and systems experience than me tell me if it's common to have some percentage of a drive or data written as all zeroes or all ones? (as in pages/clusters, or larger blocks)
If so, making the controller reckognise pages of all zero or all ones and not write them at all but simply list them in the LBA table with a bit (or a few bits) of metadata indicating this would allow for freeing up space, reducing overhead, and always saturating the interface for such chunks of data.
Even more effective if it's a significant percentage, like >1%, having a custom driver reckognizing blocks of zeroes written and simply telling the controller "these LBAs are dev/null, {block list}" would avoid taking up bandwidth for data that doesn't have to be written, and allowing f.ex. empty container files consisting of only zeroes (or ones) to be written near instantly.