[citation][nom]Shin-san[/nom]I've been disappointed with the 4k performance of hybrid drives, where the SSD is needed the most for them. I get 50 MB/sec off magnetic drives on large files.[/citation]
You need repetition for the data to be cached in the NAND flash. Then the performance will be boosted. If it's brand new data, it's being accessed solely from the mechanical portion of the drive.
Both Seagate and Toshiba offer only 8GB of NAND in their hybrid drives. So I assume that they have probably tested prototypes with 16GB of NAND and found that it didn't improve performance by much. But still, it's hard for me to ignore that 16GB would allow for more data to be retained in the NAND cache and therefore 16GB would instill more confidence in achieving a more consistent speed boost from the NAND cache, even with programs and files used less often (especially with a 1TB or larger drive). I am waiting for 16GB models to come out before I use hybrid drives and recommend them to others.
Adding just 8GB more NAND shouldn't increase the overall cost of the drive by too much, as most of the cost comes from the controllers (SSD Caching, etc) and HDD anyway, which remain the same no matter how much NAND is included.