One company (I can't recall which) has an 18x drive in the works, but who knows if they'll be able to make it work well enough to release officially? 16x is generally regarded as the wall for DVDs. The drives simply can't spin any faster and still be reliable.
Rewritable speeds may increase, but I wouldn't expect anything significant.
Dual layer speeds definitely will increase, but by the time media technology catches up to burner technology, you'll probably be ready to upgrade again.
In other words, just get a 3550A.