If I understand it correctly, eSATA and SATA are running the same protocol. The only differences are 1) port shape 2) eSATA provides power supply
I would rather not use an enclosure because I may use a high speed hard disk drive but the controller in the enclosure might be a performance overhead.
Best Answer
Yes, you can. But you probably should not.
An enclosure protects your hard disk from mechanical and electrical abuse. I wouldn't use an external hard disk without some kind of protection. If you can find a eSATA-only enclosure, I doubt that it will have any kind of active electronics inside, since SATA and eSATA are the same, on electrical level.
Some motherboards come with eSATA bracket, which is essentially a eSATA-female to SATA-male cable. You can try building some kind of DIY enclosure with that cable (you'll need to supply power for your hard disk separately).