OVMS should be able to read most of that information. It says that my (new) battery is 111,2 Ah:
CHG: 3293 (~3776) Wh
Range: 58 - 58 km
SOC: 100.00% (47.46%..100.00%)
ODO: 1043.5 km
CAP: 102.9% 111.2 Ah
And that is 102,9 % of the guessed rating of 108 Ah
You can also ask the BMS about battery info, cell deviation, temp and so on.
But i suppose that Renault will do their own mesurments on the battery and can come up with anything they want.. if your not able to proove that you have lost more than 25% range, then you have a case. As far as my research goes, no batterys has been change due to degredation, only a couple in new cars that was not working properly.