Used them about 10 years ago .... very big fins with heaps of drive and hold. Bit too big for my liking.
They are very slow (quite big), do yourself a favor and get a Quobba set.
Performers are also quite slow, with a mediocre foil profile.
BUT, the Gerry Lopez rears are nice because they are the smallest rears in the FCSII line, and also they have a symmetrical foil, that gives a smoother rail-to-rail to quad setups. So, if you want a tool-less fin set, you may want to pair them with better fronts. I used to have Al Merrick L fronts with Gerry Lopez rears before switching to Quobbas.
For instance I used Al Merrick twins in front with GL rears for micro waves: (i would switch to normal AL fronts in small waves). Al Merrick fins have a more refined foil than the GL, but the AL quad rears were too big for my taste.
What size Quobbas are you using ? Are the large too big ?
At 97kg I use mostly large fronts with medium rear(s). But the first year, only the M (orange) were available, so I used only Ms.
M are faster, but can lack a bit grip for my weight in slow waves, or when the tail encounter the foam.
You can see a lot of Quobba setups from me and others in their thread: www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/Quobba-Fins-?page=6
are the Quobba carbon worth the extra money
Given that they are only 6% more expensive I'd say yes, if you are looking for a more precise feel.
Worth the money when buying new ones, in my opinion.
Is it worth replacing existing Quobbas with Carbon ones? It depends:
I upgraded my Quobba to carbon on all my "vertical surfing" boards (tight turns in the pocket), but kept the glass ones on my "horizontal surfing" ones (carves, open face manoeuvers) where I don't mind a bit more flex.
More details in my post: www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/Quobba-Fins-?page=6#2667554
Used the Gerry Lopez Quads a few years back. I found them big and slow (as described above) with a little too much grip.
Moved onto Medium FCS II Performers for a while then finally found a set of FCS II Reactors (Medium again...). To begin with I could only find a Thruster set which I used with the Performer Rears but recently found a set of Quad Rears in the Reactor line.
This has been a bit of a game changer... far more reactive. Turns harder and faster with just the right amount of looseness.
I'm 95Kg on a bad day and find the Mediums a good size.