Better, but still very low... Assuming a constant 20MB transfer, it would take more than 4 hours to read a whole 300MB disc. If 20MB is 1X, i guess things will improve but still...
By what math does 300/20 > 14400? I'm assuming you meant 300 GB disc... Either way, what does that say about 400 GB hard drives that have 35 MB/sec transfer rates? In practice, for this to work as a consumer WORM medium, you'd have to make it natively multisession.
One of the things with holographic media is that if you develop a material that holds up better data fidelity over multiphase writes, then you gain better density and throughput for free. Of course, your throughput rate is going to grow at the same rate as your capacity, so it doesn't really save you any time.
Still, it would make at least a little bit of sense for a while at the hands of software publishers and content publishers who can setup overnight build processes that burn to disc, so they can afford 4 hour write cycles.