I have a 3 X 8 drivetrain with an 8 speed chain and 9 speed derailleurs. My shifters are not road shifters so I do not have any trim capabilities. There is a little rubbing in either the highest of lowest but it is so minor I don't even bother trying to eliminate it.

The only time you will have to trim is when you have the derailleur set to run in the biggest cogs and you have worked your way down to the smallest cogs, or vise versa.

My advice is: get a 9 speed - generally 9 speed derailleurs start at a slightly higher quality level than 8 speed (sora), and the advantage of the slightly wider derailleur will be lost after the Sora begins to wear and get a bit sloppy - which happens a lot faster on Sora than 105 or Ultegra.