My C172 is experimental due to a V8 engine, so I can use non-cert parts. I can not find specific FAA guidance regarding using experimental instruments or EFIS systems and filing IFR in IMC. the experimental equipment manufacturers seemed to be overly vague when asked if it would be legal. The only FAA regulations I could find simply say the equipment must meet the same requirements as certified.

specifically, my question relates to using GPS as a primary navigation for approach. I reference that certified units need a RNP alert or method to alert if navigation performance falls below a certain accuracy, such as .3, and deflection of flight director is more precise as altitude decreases, but if a manufacturer states that their unit meets same standards as listed in TSO for certified aircraft, is that enough for use in EXP aircraft? Would the pilot have to document such accuracy and compliance on his own?