Well it doesn't out perform the 15mm rectilinear lens, which it shouldn't.
I myself have the Sigma 18mm f/3.5 with an added filter ring manufactured locally, this was manufactured around the same time as Nikon were manufacturing the AIs 18 3.5 lens and was intended to take sales from Nikon, which in my case it did.
I have had the Sigma and Nikkor versions alongside each other mounted on my two F3 HP bodies. Even through the viewfinder one could see an apparent contrast and sharpness difference over the Sigma lens.
With film you could also see a slight difference on as best as I could get identical testing conditions. The colour prints were slightly different in colour bias, which is to be expected, but the Nikkor was certainly better.
In B&W there was only a slight contrast difference, to our eyes that is. The our eyes were the two owners of the lenses. Me the Sigma and my friend the Nikkor.
My Sigma cost me $350.00 the Nikkor cost my friend around $1,400.00 new.
The Nikkor is an extremely good lens, certainly better than the Sigma of the day and the Sigma was probably the best of the aftermarket versions available.
I enlarged, for close to 5 years, B&W and colour prints from a couple of photographers who both had and used the 18mm 3.5 Nikkor, almost always they were very good to excellent.
Mick.