This doesn't directly answer your question, but I would suggest you reconsider the CPU-driven encryption. The main reason is obviously security - while the encryption provided by the drive might be fast, I don't think there is any guarantee it is as safe as well implemented drive/partition/file system encryption on kernel level. No matter what the manufacturers tell us, one cannot reasonably exclude the possibility of the encryption being back-doored. If you use an open-source solution, you can check for yourself (or have someone do it for you).
As far as performance is concerned, unless the usual workload is copying gigabytes of data to and from the drive, the software encryption overhead is likely to be acceptable. Apart from other things, caching is also likely to alleviate the problem (whenever plain-text can be cached, of course).
That said, the main reason for setting your HDD to use encryption is that nobody else does it before you do (thus effectively trashing your data) - I believe there usually are some protections in place when changing the hard-drive password (as opposed to setting the first one).