I too have been trying to get SH3 working in widescreen. I've never played in widescreen before and since I have a widescreen laptop, decided to try.
I can't get anything to work.

I'm trying the MaGui WS mode. I tried changing the resolution of my screen and the resolution in the d3d9.ini but that didn't work. It makes the game load in the lower left corner of my screen and shrinks it. At times I get "cannot initialize 3d engine" msg.
I play on my laptop which has a 15.6" 16:9 WS (1920x1080) screen. I would prefer to get rid of the black (overscan) bands on the side, but I can live with them if I have to.
I think I've tried everything mention on the subject in SubSim and nada.
Thanks,
Mike
15.6" 16:9 WS (1920x1080) screen/8GB RAM/AMD Radeon HD 6990M video w/2GB RAM/Win7 64bit