My view point on this situation is this. The loader was built for a certain load limit. It will probably lift more, but unsafe things will happen. such as bend or breaking the front axle. I believe the specs are at the pins where the bucket is hooked . The closer you get the load to this point, you can lift the limit for the tractor. Pressure is what provides the force, and if you have a good pump, the pressure will be the same through out the rpm range. The rpm provides the GPM's which gives you the speed of operation. If your FEL quit lifting, then the full pressure was not getting to the cylinders. It could have bypassed in the pump, or by the spools, or the pump is worn. I had this situation, and I could only lift about half the rated load, and thought the problem was valve, but it checked good and then rebuilt the cylinders. I finally replaced the pump, and I am now lifting the rated load.
Your valve will only relieve when the cylinders can not handle any more fluid, and the pressure builds up until the relief setting is met. If the relief is working, that tell you that the pump is operating close to max. If you put a gage in the hyd circuit, you can see the pressure readings.