There are several models of the Sigma 28mm f/1.8. I own the last one, the 28mm f/1.8 EX DG, and these comments are based on that lens.
AF Speed and accuracy on an F100 -- The Sigma is at a disadvantage here for a couple of reasons. First, at least my version focuses more closely than the Nikkor, which means the AF system has to work harder over the common range. Second, it's a "screwdriver" lens instead of one with a built-in motor. Slightly OT, but since you mentioned "my digital stuff", you may find it interesting to know that my Sigma had to be rechipped for full compatibility with my dSLRs. Depending on which model you got, this may or may not be possible.
As for so-called "IQ", there's no gentle way to say this. On 35mm film, my copy is soft in the corners until you stop down about f/5.6, and somewhat soft in the center at f/1.8 to f/2.5 or so. By all accounts, the 28mm f/1.8G is a significantly better lens. I would attribute this less to brands than differences in price point and technology over time. More recently, Sigma's been putting out some very nice lenses that more than give Nikon a run for their money. But had I been able to sell my copy of the Sigma for $200 or so, I'd own the 28mm f/1.8G today.
The short answer is that in theory it's possible to adjust the Sigma for optimum performance with your F100 at a given distance. However, given that such adjustments are usually made by taking a series of shots and comparing the results, the cost could easily exceed what you paid for the lens, or even that of a new 28mm f/1.8G.