[canjobear]

German final devoicing is in no way an argument against phonemic theory. You're right that some experiments have shown that it is possible to phonetically distinguish the final consonants in Rad vs. Rat. However, all this means is that the phonemic /ra:d/ is spelled out into [ra:d̥] and /ra:t/ into [ra:t], where [d̥] indicates a lower voice onset time than [t]. It still remains the case that the pronunciation is fully determined by the phonemes, without reference to the spelling or other factors.