What features are "of use"? No joke here, serious question. The single firmware upgrade I ever performed was to enable TRIM on an ancient Samsung drive, back in 2010. I can't think of any feature that might be added to an SSD with a firmware patch. This drive died days later, BTW, suffering from unrecoverable hangs.
Well, actually this is a textbook
secundum quid 
Actually, the better manufacturers provide in-OS software to monitor the SSD, blank it, tweak it and update its firmware. Intel has its well known SSD Toolbox, Samsung has their Magician, Sandisk has their SSD Dashboard, etc...
No firmware out of the factory is perfect, let me see, a few examples I have in my head:
(1) Corsair Force 3/GT - serious firmware issue out of the factory leading to dead drives, a few firmware updates were issued very rapidly and sequentially.
(2) OCZ Vertex series - same types of issues, immediate firmware updates to users.
(3) Intel 520/530 series - very reliable series with incorrectly set usage counters in the firmware, such that it always reported very low percentage wear, fixed with a firmware update through the Intel Toolbox.
(4) Samsung 840/840 EVO series - performance degradation to about 1/2 to 1/3rd of fresh performance due to TLC algorithms they employed, at least two firmware updates were issued.
(5) The infamous Nvidia chipset + Sandforce SSD incompatibility - most manufacturers did not bother to release firmwares addressing this issue, which was technically Nvidia's fault, but some tried to allow their SSDs to work on certain Nvidia chipsets, I am not sure if any of those Sandforce drives still work on Nvidia MCP-equipped Macs.
So I think there is good reason to update the firmware, the same way you would let the operating system update itself. In fact, Apple pushes out firmware updates all the time, as well as HP, Dell, etc., through their integrated software updates.
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I put a Sandisk ultra II 960 in mine (late 2011 17") four weeks ago, but never thought about firmware upgrade issue, especially seeing that I put in in the optibay. I also did the 16G ram upgrade (Crucial 1600mhz) and the sys report shows the SSD link speed as 6G, while the hd links at 3G in the main drive bay.
So far I've had no stability issues or crashes and the speed seems to be as good or better than our late 2013 i7 Air, even though speed testing gives highly variable results from 140-480MB/s on the Sandisk. I just bought these off amazon after reading many, many reviews and questions for these items, but not the tedious Windows based ones. I'm surprised the extra ram didn't make a big difference in your machine, but before getting these, I had 8G in mine and it was a total dog, even slower than my Cube. Turns out one of the chips failed the hardware test but I never thought to try it as it booted normally and never crashed.
The Ultra II series are TLC drives with SLC cache. If you write to the drive past a certain amount, the fast cache gets used up and the speed drops dramatically. Perhaps your testing has revealed some of this behavior ?
I have the same drive, by the way.
