First off, the B2601 is a great tractor! I was seriously surprised with the amount of work I was able to accomplish with that tractor! I will now and continue to recommend it to neighbors, friends, coworkers and forum members.You may not get all that much benefit from an L2502 over a B2601 anyway. More tractor, yes. More work done.......not so clear.
I'm surprised at the price to change, but perhaps your B doesn't have a backhoe?
On your questions re: backhoe power, and ignoring some of the detail, yes in broad terms you're right. The backhoe on an L2502 is using only a portion of the power of the engine, it won't bog down no matter how hard you use the backhoe. The pump can only pump so much. And I think the pump on the L3902 is the same as on the L2502, so the extra HP is doing nothing extra for you. As to whether it's 10HP or 15HP that the pump is using, not sure. But it's not 25HP.
However, speaking from my personal experience working on my property, Yes! The L2501 / L2502 is a worthwhile upgrade from the B2601 and one I would recommend and seriously consider.
Here are the two beasts together!
I use my tractor to grade and level land, till dirt, move dirt, move brush, skid logs, transport trees and logs, make trails through the forest, maintain our gravel driveway, remove stumps, run a PTO powered wood chipper and to load and unload miscellaneous materials and products from delivery trucks.
The B2601 simply lacked the lift capacity, the size and the weight required for removing medium to large stumps, carrying logs, carrying stumps and the heavy ground engagement work that is needed on my property. The L2501, has been able to perform all the tasks the B2601 struggled with, or could not accomplish, in addition to getting the work done quicker.
Depending on expectations and the tasks the OP has planned, the L2501 / L2502 could be a very worthwhile upgrade.
Mike