Yup, that's it. I also have a 52x burner, and Nero only lets me burn at 32x or 40x maximum depending on what type of media I put in. Unfortunately, I've never seen 52x rated media around (only 48x).
This is very eviden with CD-RW. The drive is able to rewrite at 24x, but if I insert a 4x rated RW disc, Nero will only let me burn it at 4x max.
Anyway, at 40x you burn a full disc in 2.5 minutes... what's the rush? 😉