I've always found Unique to be a little sootier than other powders in the same burn rate range. Fiseyou, I would up the powder amount a bit. 5.4 gr of Unique is below the minimum recommended by Speer (5.5/6.5) and well within the lower 1/3rd of the range suggested by Lyman (5.0/6.8). Checking my Hornady #4, I'm reading a different range than you quoted. They say 5.3 min to 6.2 gr max for Unique; so you are low in their range as well. I believe your assessment of cases failing to seal is correct.
That being said, if you like the rounds you're making, I see no reason to change the load. A little soot doesn't hurt anything as long as you don't mind the little bit of extra cleaning it might add.
Are you using a carbide sizing die? If so, why are you lubing the cases at all? You can take that out of the equation altogether.