Tough one. I haven't lifted rocks, but I've lifted some exceptionally heavy stumps. In either case, it's less of a "lift" and more of a "controlled drag".
While a thumb is infinitely helpful, my reason in adding one would not be to pick up bigger rocks with the BH than can be lifted by the FEL. Generally speaking, I think I'd call the FEL far more capable of heavy lifts. Remember, the ~500# rating is way out by the bucket. If you use straps and hang something off SOME OTHER POINT on the FEL, you can get a good bit more lifting capacity.
EXAMPLE:
I needed to move a bag of sand left over from my patio project. This single bag of sand (delivered by flatbed and truck-mount forklift) started as 1,500# and had probably 700# to 800# remaining.
Hooking straps to the top lip of the bucket and running them around the FRONT of be bucket was useless...I couldn't lift it. And even with the BH attached, the rear end was coming off the ground. Very dangerous.
So I ran the straps the other way, hooking them to the front lip of the bucket and running them around the back of the bucket. That one change (effectively moving the weight back ~18" on the FEL arms) allowed me to very, very gingerly drag it along the ground to an out-of-the-way storage space. I could even "oomph" it onto a wooden pallate.
Several ways to skin a cat. Get a thumb to help with a myriad of other things, and maybe it'll help with a troublesome rock. If you purchase to SIMPLY haul around larger rocks, you may be a little disappointed.