The prior posters are correct in that for whatever reasons, SH5 (and 4 too) are more GPU-demanding than other high-end graphics games.
Check the temperature - your card may be getting too hot. I have a fully water-cooled system and my 2-year-old 260 runs SH5 butter smooth at max settings, 1900x1080 (Samsung 37"
). That is, except for a little bit of hesitation mousing around in the control room which seems to afflict everyone regardless of GPU. Shadows on/off makes no difference in frame rates.