I have the same as GMann, Superdrive that burns @ 16x for CDR.
The combo drive is 32x. That's fast!
From apple's site...
Combo ("Combination") drives (DVD-ROM/CD-RW) have the capability of creating music and data CDs and reading DVD disks. The drive writes to CD-R at 32x and CD-RW at 10x, reads DVDs at 12x, and reads CDs at 32x. The drive supports CD-ROM, CD-Audio, CD-R, CD-RW, CDI, CD Bridge, CD Extended, CD Mixed Mode, Photo CD, DVD-Video, and DVD-ROM media.