Forwarning: I'm not shaking my fist yelling.. Just giving some feedback.
I'm pushing an I7 at 3.7ghz w/ 8GB DDR3 and dual Radeon 5870's and creeping down to frame rates in the 15's and 20's during certain stretches even at 1920x1080.
20 FPS here with 1600 assets on the tile.

What further confuses me is the tiles aren't exactly packed with assets either. 1600 assets on the tile in that one above.
For comparisons sakes
42FPS with 3500 assets on the tile. (WCML)

There is really something a miss here.. 1600 assets on a tile shouldn't be sending my rig to its knees. 1600 assets on a tile is technically speaking a low asset count.. Did you by chance set the LOD on all your assets to some insane distance? It's the only thing that can explain this. Something needs some serious optimization and looking over.
I can roll through WCML (which is known as a performance killer) at 5760x1080 from end to end never once dipping below 35fps.. Your route at 5870x1080 is a slide show in certain areas (around the city) yet has 1/2 the assets on tiles. Once I get away from the city area the play is fine, but your entire city area is crippling.