I'm really surprised by that. Are you running really big networks? I'm playing a sandbox in Central City with 56 lines and 200.000 passengers transported per day and running at about 25-30 fps. Changing graphics settings has no impact on performance. Default ruleset. Are you running networks that size silky smooth?
Interesting.
Only framerate drops I've had are at this one transfer point when there is a several hundred people on-screen at once. The system definitely isn't 56 lines, but a decent sized system that covers most of the city. On my mac side I would probably get about your FPS range in your exact situation. Windows side(Windows 8), though, after the lastest driver updates, my GT 650M just screams and makes short work of this game. I would try installing the latest drivers. Other than that, I can't think of why your desktop GTX 670 and desktop CPU would have performance like that.