You cant really know 100% just from playing a long time >.>, I would think it should be higher, even if its not noticable, but still other then exp whats the point of uniques lol, normal mobs might have like 0.001% chance(for exmalpe >.>) then a unique could have 0.005% its still higher but the chance for seal items is so low to begin with its not much of a diff, also the fact uniques drop like 20 items at a time >.>, each possibly could be an sox, so that makes it higher too ;p.
I bet it seems to everyone like normal mobs drop more since we kill countless of them and get the items, and kill very few uniques and get nothing, but when you barely get the chance to kill uniques and you do actually kill alot(alot as in 20+ or so?) and still get nothing it seems you have been screwed lol. But the fact they only spawn once every few hours kinda makes it alot lower lol.
But I dunno anything 100%, it just seems like it would make alot more sense to make the boss mobs have higher drop rates.
Point is for normal mobs you can kill like 1,000(thats lower then what it would really be too i'm sure

) and get 1 sos, if that.
So even if the rate was doubled for uniques, you could get 1 sos for 500 uniques, and and like I said, you barely get the chance to kil uniques so you definatly arent gonna kill enough to be able to determine if its higher or not.