the experiential perfect with this verb can be expressed by both (he has watched it, the experience lives with him, but he isn't denoting that he completed it recently... which would lead me to use the imperfective)
Not sure this works.
I wouldn't like to rain on your parade but these two seem to be absolutely equal to each other by meaning and completely interchangeable:
Я только что посмотрел этот фильм.
Я только что смотрел этот фильм.
The only tiny nuance is that "смотрел" denotes slightly increased duration of the action (watching) than "посмотрел", which simply states it as a fact.
Another difference between them is that "смотрел" does not refer to whether the watching has been completed or interrupted (prolonged action) while посмотрел clearly states that you have been done with the film (momentary action).