I'm using a naughty mini with 1.8ohm coils. I have found it nice around 12.5w (4.7v) maybe 15w (5.1v)with some juice.
I could be wrong but i seam to remember reading that the istick can put out more power than the screen says. I'm sure more experienced people may pop in with views on this.
If i was you i would stick at the 4.6v setting or maybe try a pack of the 1.6 ohm coils see if that's any better for you.
Exactly right my egg, flour and milk based savory baked good friend.
The iStick is overpowered and will be delivering much more voltage and / or wattage than is displayed on the screen. For an equivalency chart
go to this thread
Add to that, the method of priming the coils, the draw technique and even the exact batch of coils you have will all make a difference to your experience compared to others. I can barely vape my Nautilus at 11 watts on my iStick as well - it doesn't bother me that much as the vape is decent.
I wouldn't worry about it, there are so many variables with vaping, just try to find a vape that works for you.
If you are adamant about getting more out of the Nautilus, there are ways, but it might mean drilling the air holes or other such drastic measures.