Hi Mariusz,
Certainly very interesting investigation and commendable results
However, I would question usefulness of long subs with an 8-bit camera - bright parts of galaxies or nebulae will saturate fairly quickly - you can see that clearly in your nevertheless quite impressive image (check the galaxy's core).
Therefore, although it might be usable on some objects and will surely provide valuable experience, 8-bits will most likely be insufficient in capturing faint nebulosities and structures without saturating brighter parts of DSOs.
Having said that, I look forward to seeing more of your space images!
Slawomir