Based on my esperiencies, the best intensity size is near values obtained using this formula: (num compute units-2) *16 that for a 570 card is 480. Try also 960 for 8gb cards.
I am using a moded 570/580 and I got 30-31MH mining eth, but in this algo I only got 600H even I tried different intensity but the highest hash I get is 670H, I am currently using 997 intensity.. I will try yours.