During my lens tests, I also experimented a bit wih mixing apertures and iSO settings and here are some first results.
First a fairly easy target. Although situated in the bright eastern sky with a lttle town a mile away, the core of M31 in Andromeda has now become readily visible in my 10X50 finderscope:
7 images stacked. All 20s, f/4, ISO2000
Below M31, totally invisible in my finderscope, I managed to capture M33 in Triangulum just barely glowing more than the bright eastern midnight sky. Thus, it also took some more agressive post-processing to bring this feeble glow forward:
Stack of 10 exposures of 20s, f/2.8, ISO 2000 and 16 exposures of 20s, f/4 and ISO 3200
And finally, two nights later, the Milky Way band became faintly visible overhead:
Stack of 6 exposures of 20s, f/3.5, ISO 1600 and 6 exposures of 20s, f/3.5, ISO 3200.
Again, all images were taken with a K-5 and the Pentax O-GPS1 Astrotracer. Considering that the sky was by no means really dark, I think these results with my DA* 200mm f/2.8 lens are indeed quite encouraging. So, now I am longing for the dark, clear September nights.....