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.