The "officially" voiceless alveolar-palatine affricate does not exist in English. But I can clearly hear it in the sentence "Ouch that hurt" (when the computer reads this sentence and often when actors say it in the movies). I am Polish and for me the English "ouch" sounds the same as the Polish "auć" (and means exactly the same). I am curious about your opinion on this.
As a speaker of American English, I don't find any difference in the final consonant of ouch, pouch, slouch, poach, reach and so on. I might imagine someone pronouncing "ouch" abnormally for paralinguistic purposes. Using Esling's demonstration table, it doesn't strike me that the ɕ token is like English, whereas ʃ is. I'm not voting on Polish: however, the categorization of segments in the IPA is not based on Polish, or English, it's based on an independent standard. <siV> in Polish may well be closest to IPA ɕ, but it is not necessarily exactly the same. What you would really want is a panel of experts to classify various sounds, so that you can uncover the range of variation that is consistent with the letter ɕ (which English ʃ would not be).
It is possible that your perception is influenced by a contextual feature in the samplke phrase: /tʃ+ð/ is likely to modify /tʃ/ to make is less retracted, making it sound more like ɕ.
Interjections like "ouch" are often an exception to the phonological system of a language, the words tsk and tut-tut-tut even contain clicks otherwise absent from the English phonology.
Examples from German include pfui (expression of disgust, a dictionary gives me the English translations "fie!" and "faugh!") and hui (expression of admiration, like "wow" or "great") with an otherwise unused diphthong, or mh-mh "no" with a clear tone contour.