The SAT II (Physics) exam isn't bad at all. Certainly nothing to worry about. While it has been a while since I took the exam, and while I know nothing of the quality of your physics education, I remember it to be quite painless: the exam only tests basic knowledge and general thinking skills. You'll probably know all the materials; the trick is in being able to correctly apply them. It's infinitely easier than the AP Physics exam, and if you're doing the online practices then I'm confident that you will be fine. The grading process is quite lenient.
...the regular SAT, however, is another story. I never practiced for it, yet I did quite well. But scores are all over the board. I've seen some pretty damn abyssmal results. But it's the people like you, who practice and worry about it, who are interested in physics, that do best.
Good luck!