I don't question the principles of physics that dictate that with clamp-on forks you can't lift as much since it is farther forward than forks attached in place of the FEL bucket.
But that granted, I can say from personal experience that I have been able to take pallets with about 1000# off an 18-wheeler (deck is about 4' agl) with my Payne clamp-on forks and B7800 without difficulty. I think that the B7800 FEL is rated at about 1000# at full height at pivot pin.
I also removed the pallet containing the tubular steel framing members for my prefab hoop barn from an 18-wheeler and it weighed over 1200#, but that, I admit, was marginal; i.e. the FEL couldn't have lifted that pallet back up to the truck deck again, or even much more than a couple of inches off the ground.
The real issue for me has not been lifting capacity, but making sure that I have enough weight on the rear to compensate. My rear tires are filled, but in addition to that, if I am lifting a max load, I put the BH on, load the heaviest boulder I can get in the bucket, curl under it and then extend the boom and dipperstick full out behind. With 1100# of backhoe behind and a 250# rock another 5-6' behind the rear wheels, nothing that the FEL can possibly lift is going to tip the tractor frontwards.
Bottom line is that I don't think that the lifting capacity difference is too significant for anything that you're likely to want to lift with a CUT and my clamp-ons have been extremely useful and equal to every task I have had.