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.
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).
Since the physical connectors are different, no, you can't. Also esata ( not USB ) enclosures don't have any kind of controller in them. There are even several models on the market that just have an open socket for you to plug the drive into so you can easily remove it and plug in possibly another drive.
I do it all the time, just be careful of shorting the pcb board and watch out for static shocks, or modify an old enclosure to eliminate the controller board but still make use of the power supply.
