It can depend on how "full" the SSD is. If it's 70% or above (for that type of SSD), the performance can see significant reduction.
It can also be a matter of specsmanship. Instead of varying the block transfer size, try using a single fixed block transfer size of 4196KB (or 2048KB or 4096KB). That may provide results more in line with spec'd values. Using smaller values can drastically reduce performance due to all the overhead.
I found this good item discussing some of the various detailed factors involved in SSD performance variation relative to some of what I mentioned above. It also depends on the controller, whether operations are synchronous or asynchronous. http://agigatech.com/blog/ssd-performance-secrets/
Yes, low disk space surely slows down the performance of the SSD. Generally, write performance is affected in this scenario. Usually, most of the well known brands set aside almost 10% of the total disk space in order to free up the performance of the entire drive. Some SSD manufacturers claim that an extra 10% of the storage is always added by the manufacturer apart from the claimed, to counter slow performance issues in SSDs when they get filled up.
But still this problem persists and nothing much can be done on this issue, except for the fact that keep atleast 25% of free space on your SSD.
Block transfer size was a good idea & I actually found a screenshot of TweakTown's raw data of their benchmarking for my same drive. I ran ATTO at the same values as TweakTown and my drive is significantly slower.
The claimed speed for my SSD is 450 MB/s Read & Write so TweakTown's looks a bit faster, however I'm really not satisfied through 256KB block speeds, especially for write speeds. Could there be anything else on my end to look at or is this RMA worthy? I don't think it's a SATA issue given that it goes over 400 MB/s eventually.