How many MB/sec can I expect copying to USB 3.0 enclosure with SATA drive?

USB3 operates at higher speeds than SATA can handle, so optimally you should notice no difference between USB3 and an internal drive of the same type. In my experience with my own drive, I've noticed this to be true.

As for exact speeds, SATA 2 operates at 3 Gb/s optimally, which gives a maximum bandwidth of ~384MB/s. SATA 1 operates at 1.5 Gb/s I believe. You won't get anywhere near that with a 5200rpm drive though unfortunately. 25-30MB/s doesn't sound overly low to me.

The best test would be to connect the drive internally and test it there. If it's getting the same sort of performance, then see my first paragraph. If not, then there may be other issues coming into play.

EDIT: Using HD Tune I did a benchmark of my own USB3 drive, which is a caddy providing a RAID1 of two SATA2 5900rpm drives. I'd expect these to operate at about 1.5-1.7 times better than the drive you describe, which is what I got as well. The fact these numbers line up would indicate to me that both our results are in fact capped by the drives in use, not the bandwidth provided by USB3.

It's very likely something is wrong with your USB connection; the speeds you're seeing are at the mid to high end of USB 2.0.

I'd suggest using USB Device Tree Viewer to determine what type of USB connection is actually being established with the device (ie "High-Speed" or "SuperSpeed").

There's a great description of how use USB Device Tree Viewer in this answer, given to a related question of mine.

To paraphrase, select the device you're concerned with and examine the Connection Information to that appears in the right pane.

Devices connected in USB 3.0 "SuperSpeed" mode will report:

Device Bus Speed         : 0x03 (SuperSpeed)

Devices using USB 2.0 will show:

Device Bus Speed         : 0x02 (High-Speed)