There is this excellent review article cited by MDB in 2017 on the effect of venting. Towards the end of the thread I more specifically include a link to one of the papers on which the review is based, a 2006 Widex technical paper that has a graph of a group of mildly sloping high-frequency hearing loss subjects that were tested with CASPA3 words in quiet at a 30 dBHL level. Word recognition scores decreased 30% with increasing vent size.
NOTE: The original link that I provided to the WidexPro website seems to be broken (Widex locked the door except for registered providers only?). I have added another post with a link to a publicly accessible version of the 2006 paper to the thread cited below.