Gama,
I remember how my old Pentax 300 f4 would reach stars of mag 14 with colour film with exposures of 15 minutes, yours (300 f5?) reaches mag 16 in 9 minutes according to my measurements!
However, I still think you underrate Canon sensors. For example attached is an ISO400 image I made in April 2004 with a standard 300D. Its of M83 at 530mm Focal length at T3.5 (about 1 stop faster than your lens). It was made under good conditions from my backyard in the bayside suburbs of Brisbane. Its noisy, but its just a single 90 second exposure....
Terry
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