One of the answers may have to do with this:
And the other thing we’re going to be fixing is, many of you will have noticed that we don’t use all of the video memory on your video cards.
It turns out that’s because of a very old bug that plagued us a long time ago and we sort-of arbitrarily, in order to avoid tickling the bug, we capped how much memory we would ever use.
We are now in the process of fixing that so it now measures how much memory is there, and we use what we think is a reasonable share of that memory.
It turns out it’s not a good idea for Second Life or any application to try to use all the video memory on the card, because that in fact will cause the rest of the system to get screwed-up.
Oz Linden
July 2nd 2014
One other thing is that the Linden viewer has a bunch of upgrades for textures that have not arrived in Firestorm yet so it may be worth gritting your teeth and using the Linden viewer for a while to see if that is any better. The next Firestorm release should have those upgrades.