I disagree. Simulations will provide good results. It is not like a simulation result and real measurement results will differ a lot. Even amateurs can make decent calculations. Experts can do really precise calculations. I don't think engineers at Dassault or LM will have any problem calculating the RCS of naked KAAN very precisely with the tools/software at their hands.
Lets agree to disagree then.
Yes simulations + predictions are getting lot better, these will still always have some number of noticeable error/uncertainty ranges given number of assumptions and asymptotic issues that are present in the transforms for say Nearfield to Farfield modelling from what I remember from my last deep delves into the subject. These can vary from small to large depending in the real world conditions too (where assumptions break down).
Especially when you make the geometries more complex and also increase the incident wavelength closer to the size of the aircraft itself (given the resonance issues). Or the "tricks" the source country may opt to include inherent to parts of the geometry details to give more characteristic signature that would not be captured by sims that do not have access to this info when constructing say the overall mesh and assuming homogeneity/isotropic there instead.
Nothing beats final RCS signature and final base dB level verification and also verification of what happens with your own radars regarding it, that will be kept close with the country and also serve as reference to how much later RCS reduction (with things like you mentioned earlier with the RAM) helps further.
That's the reason for continued luneberg lens use (past training reasons etc) by say F-22's flying around kadena/okinawa even though they know the Chinese could predict where the RCS range is to some decent enough degree....and test the longest wavelengths onboard their destroyers etc. i.e Preventing exact readout of exactly how low it actually is and stuff you have tinkered with on top (with say the longer wavelengths in mind that get better at detecting this size aircraft to begin with).