Well, I came to the conclusion that this thread is not the greatest, 19.5 is not so different from 19 or 20 and it has been discussed many times over, but I do have something to add. SROG'ers may benefit from more than 18 hours a day because this makes the plants stay short, I have noticed that plants stretch at night. Apart from the short plant it also allows more time of light to the lower side-branches, this is great for SCROG'ers.
I've seen this mentioned several times. Since the plants aren't getting a 12 hour uninterrupted dark period (which is the key to flowering), they stay in veg. I haven't done it myself, but it seems to make sense in theory.