Lens Suggestion for City Scapes
If you want faster you are gonna probably end up with 2 lenses to cover the same range.
However, the faster lens might end up not showing its advantage on city scape where I assume you will desire a larger DOF.
The 17-55 may be the lens to look at though. Perhaps just keeping the 18-200 for your longer shots.
If you are determined that the apeture is your limiting factor on the 18-200 then I agree with you, the 15-85 makes little sense.
What is your intention with combining the faster lens with shooting city scapes? Perhaps this will help shed better light and offer a much more accurate recommendation.