Strangely, i don't own top end GC (only my brave old Zotac 580) neither top end proc (only I5 3470 @3.2 GHz) but can say my city battles are at least 80% fluent.
Yes, only 20% battles where sometimes i can get occasional slide show, but changing the zoom while stuttering often solve it.
Again strangely, i have to zoom somewhere instead of full de-zooming action!
Aready got 40VS40 units without slide show (i recall a fantastic battle at Bibract city, completely crazy and indecisive to the end - 50 minutes!)
On side note, i run Rome II at 2560X1440 reso, i'm playing large units size (not ultra), and only my game units settings are on ultra. Others settings are on High, except particules set on low (see spoiler).
I have SSAO on, but no antialiasing (but FXAA on via Nvidia CP as well as anisotropic X4 and triple buffering)
I have the unlimited memory box checked too.
My SweetFX also enhance the whole game nicely with cristal clear picture.
Very satisfied so far with TWR2 graphicals (tried lately NapoleonTW, but was very desapointed)