There are several problems with the "firmware is the cause of lower performance" theory:
What is the most costly component used in a SSD? The NAND data storage chips.
What component used in SSDs has decreased in price, causing SSDs to be cheaper? The NAND data storage chips.
What component used in SSDs has evolved from using an asynchronous (one data transfer per clock cycle) data transfer at a 66MHz clock rate, to a synchronous (two data transfers per clock cycle, as in DDR memory, Dual Data Rate) data transfer at 100MHz, 133MHz, and 200MHz clock rates (the latest types up to 400MHz)? NAND data storage chips.
What component used is SSDs was said to have a shortage of availability in 2013? NAND data storage chips.
What would be the easiest way to save money when purchasing NAND chips? Buy old stock asynchronous NAND chips running at 66MHz that virtually no one else wants.
The argument that many people won't know the difference between a poor and normal performing SSD is a good point, but the poor performance could easily be caused by asynchronous NAND. IMO it is much more likely to be the type of NAND used rather than the firmware.
One of the major selling points of the SandForce controller, as sold to SSD manufactures, is the manufacture has very little (if any) design and engineering work to do themselves. That is, the manufacture uses the "reference" board for the SSD, with the SF controller and associated components already engineered and tested, including working firmware (that's another story). All the SSD manufacture must do is choose the NAND and adjust a few things for each type of NAND, probably using the SF design documents.
Very few SSD manufactures using SF controllers modify the firmware much if at all, and I doubt that Kingston is one of them. The main one we know that has modified the firmware is Intel, and AFAIK the other is SanDisk. If Kingston is using "5.06" as their firmware version number, that exactly fits the official SandForce 5.0 firmware release line, which was 5.0.0, 5.0.1, etc. IIRC, SF firmware 5.0.3 finally fixed the malfunctioning TRIM bug introduced in the new SF 5.0 firmware line.
Assuming that Kingston modified the SF firmware, and their 5.06 firmware causes poor performance, why not just go back to the 5.05 firmware? Why would they even need a new version of the firmware, if the only difference is the location of the factory assembling their SSDs? One answer to that question is to accommodate different NAND chips, like asynchronous.
Do we have examples of very similar SSDs that differ in performance due to the NAND chips they use? Yes, the OCZ Agility 3 and Vertex 3.
IMO, the OP virtually proved his point (and mine) with the quote from Kingston stating that the NAND rather than firmware would cause a difference in performance. To many details add up add up to the use of asynchronous NAND in some production runs of the V300.
I also see a different clue in the OP's statement that, "every case I've seen so far has been the 120GB capacity". With the shrinking of the NAND to the 2x nm level and less, it's possible to put 60GB of user space on one NAND chip, or possibly even more. The potential problem with that is the number of channels that NAND chip has to connect to the controller does not increase, due to the physical design and standards of the chip package. Larger capacity SSDs with more NAND chips can use all the channels the controller has, but the 120GB V300s may be limited to the use of four channels instead of the usual eight. That is pure conjecture on my part.
Believe me, I would hate it if I had purchased one of the "5.06" V300s, and I get no satisfaction from writing this. I wish it was a firmware issue, but IMO it isn't.
I'm sorry, but I'm to lazy to Google that for you...