Will FileVault reduce the power of my hybrid SSD+HDD? I have a MacBook with a Momentus XT hybrid SSD+HDD drive. It have a 500GB HDD plus 4GB SSD which, as I could check, it uses as a cache for read operations. It uses some kind of black magic to determine the most used data and let it easily accessible through the SSD storage.

I wonder if I can enable FileVault 2 to encrypt this drive and if it will kill these feature of the hybrid drive.

I know that FileVault always lead to a performance penalty, but my wonder is if the drive will become unable to determine the portion of data that can be SSD-boosted due to the fact that it will become encrypted.
FileVault 2 encrypts the filesystem at a block level.   Despite the data being encrypted the cache in your disk should still be able to work out that particular blocks are being accessed more often and cache them in the SSD.